How do you sharpen fonts?
Settings, User Interface, ensure Anti-aliasing is switched on. You can then subsequently experiment with the level of hinting and (if you have an LCDisplay) sub-pixel hinting.
Further tweaking is possible by using a special version of the bytecode interpreter libfreetype6 (software that takes care of translating a character in a set of dots on the screen). This version operates inside patents (held by Apple I believe), so is by default not included in most distributions. Special repositories (PLF for Mandriva) distribute it. I have installed this, but am not sure that I really find it giving a better result than the standard libfreetype6.