New ball joints should help, along with a steering alignment (duh), flush the p/s fluid. Other than that, you're looking at new tie-rod ends.
Tie-rod ends and ball joints are pains. It'd probably be easier to buy a new rack. Still putting a new rack in is a PITA too. I'd start with a good alignment and flush the p/s fluid to make sure its running correctly. Also check p/s pump belt tension to ensure its operating at peak pressure. However that probably won't change much unless your p/s is fundamentally broken, as p/s doesn't actually do anything except at low speed.

After that, if you really want to optimise steering you need to look at things such as kingpin inclination, toe, steering axis inclination and hence scrub radius, camber etc.