Most casual/recreational players and even some serious players sometimes forget about the most important tool in the game! If you are still using that high school racket or have left your racket in your trunk for the last few months, it might be time to have your racket tuned up.
Modern rackets are made of materials that retain their original playability characteristics far longer than older ones such as wood and aluminum. So, I am not suggesting an expensive, top of the line racket (around $200+ these days!), but maybe just a tune-up just like an older used car might need. A freshly restrung and regripped racket will play almost like new.
Have a look at your racket before taking it to be restrung. Cracks in the racket will make a restring impossible. Cracked or torn grommets can be replaced for a small additional charge. (may be order online Tenniswarehouse.com or Tennisexpress.com are two suggestions)
Finally, have a look at the strings in your racket. Do they look worn, or are they fraying? An old set of strings will even sound dead, kind of like an out of tune guitar.
Fresh strings might give your game just the boost it needs!