(Vaguely) this question has been asked before. I know, 'cos last time i answered it I got it wrong (slightly). I would suggest these steps.
- Monitor your hit rate
- Adjust config
- Monitor your hit rate
When adjust the config, look at
http://www.linux.com/feature/153221. The trouble is, most of the things that you can do to increase the hit rate described in the article ae in danger of breaking something, whether that is an RFC or your user's browsing experience. It is up to you to decide to what extent you are prepared to take that risk (and you may want to take into account whether your users are prepared to force a page reload occasionally).
Oh, and there is also adjusting the memory size appropriately, but for that you also have to monitor your hit rate...