I tend to divide DMing into three distinct skills. These divisions are, of course, arbitrary, and there is a degree of overlap between them. However, I do find it helpful. The three areas are as follows:
Rules mastery: Should be obvious - knowledge of the rules, how to apply them, and when to ignore or change them.
Story mastery: Everything to do with plot, characterisation, and so on. Basically, anything that you would find in a good novel falls under this category.
Table mastery: People skills. This is your ability to deal with or prevent disputes, to build a good group, and to enforce your authority when it is required.
Any or all of these can be improved, but the techniques for doing so are very different in each case.
Rules: This is the easiest of the three to improve, because it's almost purely mechanical. The DM can just sit down with a PHB and create lots of characters, run lots of battles, and generally test the rules, until he's confident with what he's doing. Practice makes perfect, and by testing the hard cases, the DM will become better able to make judgements on the fly. Also, long practice will reduce your need to check books during play, and give you a guide to where you might want to ignore the rules to speed play.
Useful techniques: I've found cheat sheets very useful here. When preparing an encounter, if you know there's some odd factor that will apply, write it down in a prominent place. If an NPC spellcaster uses lots of buff spells, note down all the effects that apply, in all the possible combinations (to deal with Dispel Magic and similar effects). If your players favour particular tactics, note the rules surrounding those tactics.
Story: Read lots of books, in lots of genres. Watch lots of TV, particularly shows in the genre your trying to emulate, but also good shows in any genre. (If you can stomach it, watching soaps can be helpful - the depth of the characters here is not great, the acting is poor, and the plots are very simple. However, these are all due to the extremely high rate at which these things must be produced, and a lot of the same pressures apply to DMing.) One book I found extremely useful was "The DC Comics Guide to Writing Comics" (thanks to Monte Cook for recommending that one), because, again, a lot of the same pressures apply.
Useful techniques: The most useful trick I know here is the compilation of lists. Lists of names, lists of cool words (with meanings), lists of trades, lists of trees, flowers, brush-strokes, architectural styles, colours, character traits. Lists, lists, lists.
Table: I found this to be the hardest area to develop, because it's the one that you can't really prepare for. All I can recommend is that you practice this. You might want to consider volunteering some of your time to a local youth group - dealing with difficult teens seems to be good practice for dealing with difficult gamers
Useful techniques: Keep the game flowing, don't be afraid to make a ruling and stick by it, and when things get difficult, diffuse the situation with a winning smile.
I hope some of that is helpful.