String
Tips
The playability of your racquet will be greatly effected by your racquet strings. Even novice players are able to recognize different string tensions and string types.
The Basics
Keep your racquet covered and clean. Dirt can accumulate in string holes or grommets and abrade string.
Keep your racquet cool. The car or trunk is no place to store your racquet. Even short term exposure can cause synthetic strings to 'relax' or lose tension.
When strings wear out, they stretch further, but rebound more slowly and with less force, resulting in less power to the shot. How often you should restring depends on the string and the type of player you are. String companies recommend you restring your racquet a minimum of twice per year.
Choosing a String
Choose a string based on these important factors: Your style, your racquet, court surface, how often you break strings, injuries and finally cost.
For optimal playability natural gut is still considered tops. However natural gut is very expensive, not particularly durable and it is susceptible to changes in the weather. Therefore unless you are a professional or a highly rated player there is no need to use natural gut.
For greater playabiltiy choose a thinner guage string, 17 or 18. Don't expect these thinner strings to last long but you should notice a greater playability.
If you are a string breaker try a thicker guage or a Kevlar/Technora string. Playability/power will go down but they will last longer and they add control.
If you want more spin try a textured or thin guage string for more bite on the ball. For arm and wrist injuries use a softer string that will help absorb the impact of the ball.