We all like to believe we are excellent Game Masters. I think most of us are excellent, because GMing involves a lot of hard work and prep. So any person who decides to helm a game, gets automatic credit from me as a player, simply because of the effort involved. It is also potentially embarrassing. You are laying out all your best ideas each week for your friends to pass judgment on. Still everyone has areas where they would like to improve. In my opinion the best way to approach this in gaming is to take it in small steps, dealing with one element at a time.
It begins with an honest assessment of your GMing style. You want to ask yourself where you feel the least comfortable, that is usually where you'll find weaknesses. Sometimes these are not an issue. If you and your group don't like big dramatic speeches from NPCs, then it really isn't a problem if you haven't developed this much. On the other hand, if your group loves them, but you have difficulty delivering, then you might want to practice improving that skill.
I am going to divide the game master skill set into some arbitrary categories just for the purpose of making discussion easier. There are any number of ways to slice this but these are areas I think you can take in turn to assess:
System Mastery: I believe this is important if your a game master. Knowing the system you run matters because that affects the speed and flow of game play. I don't think every GM needs to be an encyclopedia of course. As long as you know enough to run the game smoothly and teach it to new players, that is enough. However I think it is something that should improve, not diminish, over the course of a campaign. If you find the latter occurring, or you find you just don't know the system well enough, here is my advice.
Read the rule book at least twice all the way through (with the exception of sections that references chapters). Then before each game re-read the rules portion and the character creation portion. Because it comes up a lot, one thing I like to do, if the book has it, is re-read the combat section the morning of each game. After a while you won't need to read every last word, but may choose to focus on a particular area like the grappling rules or vehicle combat.
Mastery of the the Material: This includes both setting and adventure material, whether home-brewed or published. This is equally important to system mastery. It slows down the game if you don't know the details of the setting or the names of your NPCs. It is easy to get lazy on this front so it is definitely worth evaluating where you are on this every so often.
If you find you want to improve your master of material, I suggest begin with organization. If you are not organized, finding information during the game will quite difficult. So set up a proper binder, computer file or online method for organizing your campaign materials.
Once things are organized make sure you review and evaluate what you have. Look for gaps so they don't come up during play. Even though it is all your own stuff, it is easy to forget what you've written, so re-read it once in a while. If you are running published material, then you ought to read and re-read until you are familiar with it.
Performance: This can be further divided into many subgroups (voices, ad libbing, playing NPCs, description, etc). I think everybody has places they are strong and places they are weak here. I am weak on voices and on gathering steam. It usually takes me about 30 minutes to get into the flow of running a game and I am not especially fond of speaking with accents and the like.
Some of performance includes natural talent, so there will be limits to what you can do. We can't all set the scene like James Earl Jones. Still I'm always surprised what a bit of hard work and practice can achieve. Anyone can make a Lamborghini go fast but if you can soup-up a jalopy to go fast that is truly impressive.
Preparation: This is important and dovetails with mastery of material, but it is its own separate category as well. Really this is more about time you put in between games and how you use that time. It is also something that will fluctuate as you GM more. You may find you require less prep time as you gain experience, but you might also find yourself unable to prep as much as you like due to circumstances. The key here is to recognize what is realistic and work within those constraints. You definitely don't want to have prep time interfere with more important aspects of your life, but you also don't want to waste your players' time by not prepping enough. It is a balance.
If you find you don't have enough time to prep, I suggest aiming for efficiency. A big help here is to write down what you absolutely need for the next session then work on those in order of importance. While listening to music during prep can be an inspiration for some it may also be a distraction that slows you down, so pay attention to the environment you prep in and remove anything that detracts from speedy prep.
If you are simply not prepping enough because of lack of interest, it may be time to take a break from GMing and be a player for a while.
Every so often, I find it helpful to take a break and just be a player. It really does help you see things from the other side and it can recharge your batteries a bit. Another reason I think this helps is it exposes you to someone else's GMing style, which can give you ideas for running your own games.
So take a little time to examine your GMing practices. If you are happy with how you run games, then that is great and no need to work on anything. If there is something you'd like to improve, the above advice may be of use to you.