Myths (in no special order)

A few of these may sound like picking on Ken Rockwell. He can take it.

• Your digital camera body is obsolete the day you buy it.  Think about it for a moment. If your camera takes great photos today why would it stop doing so tomorrow, a week later or ten years from now? It's persistent marketing and the human desire for novelty that breeds the notion of obsolescence. Do you really need face detection or is it just something you have been persuaded you need? My six year old Sony DSC-V1 snapshooter still takes fantastic shots just as it did on day 1 - in part because it has "only" 5 megapixels and is noise-free. The excellent Zeiss optics don't hurt either.
• Your computer is obsolete the day you buy it. This durable canard just won't go away. It's more marketing ploy to get at your money. The essence of a computer is its ability to mimic other sorts of things and to accommodate new technology.  My oldest laptop is a Sony VAIO purchased in 1996. It's still a great machine although it runs the "obsolete" Win98/SE and has "only" 128 megs of RAM. It has a beautiful monitor that continues attracting favorable comment. Get a good unit with expansion capability and it will serve for many years.
• Any cheap film camera can beat a high-end DSLR. Well ... in my experience it is just the opposite. Practically any cheap digital pocket camera can beat the pants off all those compact "One Step" (Nikon's sad offering of the time) 35 mm junkers foisted on us in the '80's and 90's. A good digi will easily challenge any 35 mm film camera at any print size. Postage stamp size images on a computer monitor are useless for making comparisons.
• A Big Box discounter like Costco can make better prints at lower cost than you can with a decent ink jet. This may be true in some parts of the world like California - center of the technology universe - but not around here in the sticks or most other places. I have seen plenty of wretched local samples. Ditto for film processing. Do it yourself or it is sure to come back scratched and foggy. That's life in the real world where most of us have to live.
• JPEG is just as good as RAW/NEF. Not for fine prints or full-screen monitor presentation. I can see the difference in every comparison I have ever made. JPEG is especially prone to losing detail in the shadows, for some reason. It also blocks up intense reds and most oter saturated colors, yielding a paint-by-number effect. That said, however, JPEG can be very good indeed and for many forms of presentation it's often more than good enough.
• Stay below F/16 or F/11 or diffraction will ruin your images. Unlikely. What if you really need depth of field? I have had fantastically sharp results at F/19 or greater. Of course, be aware of aperture and don't stop down more than you need to but - there may be more to taking a great picture than avoiding diffraction. Don't be a fanatic about this.

Tips

While some of these may prove unique, you will find many of them at other sites but having them collected here may prove useful.

• Always use a lens hood. It's not just to keep the sun from striking the front of the lens directly (you can always prevent that with your hand) but to limit any sort of light from outside the picture subject area entering the camera. This extraneous light bounces around inside the body contributing to flare and slightly fogging the image. The effect is visible, especially in bright sky landscapes, and detracts from both color saturation and contrast.
• Put away the sandpaper and steel wool. It takes a lot of dirt on the lens to begin affecting image quality. If you must scrub-a-dub-dub, blow the grit off first with compressed air and hope this is all you ever have to do. Glass is actually a rather soft material. Folks who incessantly polish their lenses gradually accumulate a fine haze of tiny scratches which inevitably diminishes image quality. This damage is permanent. Best to clean very infrequently and very carefully. I have film camera Nikkors that have not been cleaned in 30 years. The secret lies in using lens caps for their intended purpose rather than just as novelty coasters on the coffee table.
• Prone position. Between lenses while exchanging, orient the camera body with the lens opening down. It's more natural to let the body lie facing upward but this is an open (literally) invitation for stuff to fall in there where it will soon seek out the sensor and glue itself into place.
• Leave burst mode engaged. It's very easy to take single shots even with burst engaged simply by taking your finger off the shutter button in timely fashion but, if you suddenly do need burst, it's instantly there. Explicitly having to engage burst usually involves fiddling with a menu or a button whose location you have forgotten. Meanwhile, Big Foot disappears into the woods and you are just another kook with no evidence.
• Flash is your friend. It's not just for when there's not enough light to take a conventional shot. Flash can be very effective at moderating harsh contrast in full sunlight, lifting blocked up shadows out of the murk, balancing color, controlling background visibility and freezing action. Use more than one for really spectacular results. Flash is also useful in self-defense against an assailant. (Watch Hitchcock's "Rear Window" to find out how.)
• Travel light. These days, with optical stabilization so common, it makes little sense to lug a tripod. You still will need one for studio work and possibly macro but otherwise, why suffer? So what about wildlife photography with long lenses? It depends. With stabilization and practice I have found it reasonably easy to get perfectly sharp results hand-held at 500 mm but the fact is, this would probably be even easier with a tripod. A good tripod - one that can actually tame the wobble in a long lens - is surprisingly expensive. If you do get one don't waste your money on a cheapy. Thom Hogan has some good commentary on that subject. A monopod is a useful compromise and works better than a cheap tripod. It's very compact (when collapsed), light, inexpensive and does a decent job of controlling the shakes on one axis, at least. Unlike a tripod, a mono has many important non-photographic uses such beating off dogs, whacking panhandlers, threatening teenagers and poking at smelly things you'd rather not get on you.
• Under-expose (with care). In high contrast situations (mountain scenery comes to mind as a possible worst case) use the camera "blown highlight" alert. Crank down the exposure bias (or reduce exposure manually) until the highlights are being properly recorded. Sure, this will under-expose the shadows but in digital photography they can usually be recovered with excellent detail in processing if you are using reasonably low ISO settings (low noise). The Nikon D300 is exceptional in this regard and I have obtained superb results by under-exposing 2 full stops below the recommended matrix reading. The highlights once blown are usually lost for good and this is one of the more distressing technical flaws a photograph can suffer from. Shooting RAW also helps. Software like Bibble and Silkypix has excellent highlight recovery facilities but still has to have something to work with.
• Stop down.  Except in the world of astronomy where things are easier there are few lenses performing their best wide open. This has to do with the unavoidable and numerous compromises dictated by things such as variable subject distance, variable focal length and constrained lens/sensor spacing. Check dpreview and you will find even the Highly Recommended prime lenses generally soft or even "dream-like" when wide open. Stopping down to F/5.6 or F/8 can cure a host of ills including vignetting, chromatic aberration and softness. Of course, controlling depth of field may have to take precedence but as a general rule the old advice pertaining to great photography still holds some water: "F/8 and be there".
• Look back. When you go on a walk to take pictures, stop and look back once in awhile. You may think you have seen everything you passed but the 180 degree reverse view could be entirely different.
• Use low ISO. DSLRs have very low noise to begin with but it's still an excellent idea to use the lowest ISO you can get away with. This improves dynamic range and color while ensuring the smallest possible amount of noise.
• Shoot RAW. If you are a stickler for image quality, this is the way to get it. JPEG loses detail where it often counts most ... in saturated colors and the shadows. It's not so much of a nuisance as some would have you believe. Most processing software has a batch mode and the conversion process with all your presets is a no-brainer. Best of all, your finest images are still there in original form for custom processing. RAW/NEF files are big but who cares? These days storage is dirt cheap.
• Avoid filters unless you really need them to achieve a specific photographic goal. It's common for sales people to recommend you purchase a "sky-light" or UV filter with a new lens purchase "To protect your lens". The real reason is that filters have a high mark-up. The best lens protection is being careful with it, not cleaning too often and using common sense. A filter has two flat air/glass surfaces. Off-axis light (i.e., away from the center of the image) from a subject travels through more glass than light that is on-axis. Furthermore, as the angle of incidence increases, so does scattering off the surfaces. These effects contribute to exaggerated vignetting. Coatings notwithstanding, those two surfaces will contribute in some way to degradation of the final image. The bottom line: use a filter if you really need it, not as an always-on precaution against your carelessness. Filters are more useful in black-and-white film photography. For example, a yellow filter will darken a blue sky which might otherwise come out a boring pale gray or white. In digital color photography most filter effects can be achieved in post-processing.
• Two is better than one. It is a real luxury having two similar camera bodies, each wearing a different lens and set up for differing situations. The advantages are numerous including backup in case of damage to one of the units and instant coverage of extreme focal length range not available in a single lens.