Joined: July 2008
I usually just lurk here, but photography's been a hobby for 35 years and I know a bit about the technical side.
Focus is always more difficult at the extremes of a long zoom because the depth of field decreases as the focal length increases (with equal apertures). Also, unless it's a "parfocal" lens, the zoom will probably require refocusing when the focal length is changed.
Some cameras do indeed allow for focus micro-adjustments. I've been playing with my new 5Dii, and have been surprised that both my Canon lenses (24-70L f/2.8 & 50 f/1.8) have required adjustment - one back-focuses and one front-focuses slightly. Normally this isn't a big deal, but when you're working with larger apertures and long lenses, it can matter. Some photographers find in time they can adjust the focus slightly in manual mode to compensate, if the body doesn't have micro-adjust. For those fortunate enough to own a 60D or 5D, there is new software called Focal that will calibrate lenses automatically while the camera is tethered to the computer by a USB cable. I'm not shilling for the developer, but he has been very cooperative during the beta trial versions.
Setting the aperture to 16 or higher will certainly give greater depth of field, but be aware that most lenses have a resolution sweet-spot between F/4 and f/11. Bumping the ISO is sometimes a better option, then deal with the increased noise in Lightroom or Photoshop.
As Lou mentions, practice makes the biggest difference in the final result. Knowing your camera and lenses well, using them until they become second nature as you work, and also the post-processing is key, after your artistic sense, of course.
Sorry to butt in - just wanted to add a few hints from an old-timer...