(The first three paragraphs of this come primarily from a response I gave in a blog comment.)
When you start out, you need to develop the fundamentals (see my article on Develop the Basics in the "How to Be a Champion" articles.) As you develop your shots, your playing style will gradually emerge. Some players have a firm idea on how they want to play almost from the start - hitter, looper, blocker, chopper, etc. Others aren't sure at first, often for years, as they develop their game. And the style often changes - I was an all-out hitter my first three years, then switched gradually all-around, with equal emphasis between looping, hitting, and steady countering, plus a little of just about everything else.
Style comes from two things: what the player does well, and what the player wants to do. They are not always the same, but they usually have a large overlap as players tend to get better at the things they want to do (because they use them more), and they tend to want to do the things that they do well, since that leads to winning. So most often players naturally develop a style based on these two factors. Others really want to play a specific style, perhaps because they saw a top player play that way. They may simply want to be a chopper or lobber because of the spectacular points they play. Or they may develop a blocking style, but simply decide they want to play like most world-class players do and become a looper. (That's a primary reason why I switched from all-out hitting to more looping.)
With my students, I regularly advise them on how their game is developing, with two things in mind. First, develop an over-powering strength, something that will dominate at whatever level they are at, and develop a style around that strength. Second, develop all aspects of the game you will use since having strengths do not help if opponents can simply play into your weaknesses. So I try to lead them into a style that will win for them. But that style also has to match what they want to do. There's no point telling someone to be a looper if he hates looping, like one of my students.
Once you have begun developing a style, you should continue to develop that style. Watch players with similar styles, learn what they do and why (this is important - don't be afraid to ask the player), and incorporate whatever you think will work for you.
Since rallies begin with serve and receive, this means developing serves and receives that work for your style. This is probably the most under-utilized, under-thought, and under-developed part of most player's games.
For example, if you have a nice loop against backspin, it might not be to your advantage to push too many short serves back long, since this gives the opponent a chance to loop and so lowers the chance of a backspin return you can loop. It also might not be to your advantage to flip, which gets you into a topspin rally, and again takes away your loop against backspin. Instead, a player like that might develop a short push, which increases the chances that the opponent will push long, giving you that backspin ball to loop. Similarly, short backspin serves will often give you long push returns to loop. And if you serve topspin, you are unlikely to get a backspin return from most players. (This doesn't mean you don't vary in these other receives and serves, just remember they are variations to the shots that should be more central to your game. For example, a sudden long push receive against some players will often result in a push return to set up your loop.)
If you are a hitter or counter-hitter who likes to get into bang-bang topspin rallies, you might want to serve and receive more with topspin. Or you might serve short backspin and follow with a slow, very steady loop to get into those topspin rallies.
Find the unique aspects of your style that give opponents trouble and focus on winning with those shots. Germany's Timo Boll, the #1 European, forehand loops with a somewhat unorthodox extreme forehand grip. This gives him perhaps the best inside-out loop in the world (his lefty loops usually break to the left), and he uses this to great effect. Much of his game is used to set up this shot, which is a primary reason he's the only European who can often challenge the Chinese. At the same time, if a shot is too unorthodox, consider whether the benefits of the shot outweigh the negatives, since the very fact that it is unorthodox means it likely has problems, or it would become "orthodox." (Sometimes the unorthodox becomes orthodox, such as reverse penhold backhands or attacking short serves to the forehand with the backhand.)
One last thing to think about when developing your style. Since so much of style comes from serve and receive, sometimes the style comes from those shots. For example, if you develop a serve that players keep popping up, you might develop a nice smash, and you are well on your way to becoming a hitter - all because of the serve you developed. Or if you have a nice backspin serve, you'll get a lot of backspin returns, and so you might develop a nice loop, and you are well on your way to becoming a looper - all because of the serve you developed. So while you should develop serve and receive to match your style, sometimes style comes from the serve and receive.
Ultimately, you should develop a personal style that's all your own, and really know your style. Given the chance, you should be able to write a book on your game; if you can't, either you don't know your game or you don't have a game. Sound familiar? See the August 15, 2011 Tip of the Week: The Book on Your Game.