That question has no answer. What is a good tension for one might be all wrong for another. It all depends on your technique and how you play.
Mag is right. Ideal string tension depends on your playing style and also the kind of games that you will be playing (e.g. singles, doubles, etc). The average player usually have their racquets strung at 19lbs to 22lbs (probably mostly at 20lbs). More advanced players ranges from 22lb to 26lbs and the pros have those above that (e.g. Peter Gade at 30lbs). Having said that, I also have racquets strung at lower tension (e.g. 20lbs) for games where I think I will be playing a smashing game (i.e. to work out my frustrations). My normal tension is at 24lbs.
The answer is as follows: work on it. Build up resistance to the stress of the high tension. The more you workout the stronger you will and the longer you can play at that level of tension.
I think a good rule of thumb is, if you are unsure of what your racquet should be strung at, is to start at 18-20lbs.... then play for a while, when you feel you can handle this, go up in increments of 1-2 lbs next time you get restrung! Simple.
