well besides maybe adding more memory depending what you have currently here are a couple of suggestions.

1. Maintenance (App) – This is a great simple application. It resets your spotlight index (making it much more responsive after all those downloads), repairs permissions, optimizes your system, and much more with just one click of the button. Tried it.. works great. I have a far more responsive system. I suggest running this once a month.

http://www.titanium.free.fr/pgs/english.

2. Monolingual – This is a must if you are looking to free up more Hard-disk space on your system. What monolingual offers is the ability to remove all the extra language support that is built-in in mac os x.

ßut the best part is that this program also removes the extra code that comes built in OS X so that it can run on any Mac. It will detect what computer you are running, and allow you to remove the extra code you won’t need.

http://monolingual.sourceforge.net/

3. http://www.thexlab.com/faqs/maintainingm – this website provides many different tips on things you can do to clean/maintain your iMac. ßest of luck and hope this helps you out.