Like JoeyR said above, upgrading the RAM is a great way to make your MacBook faster for a reasonable price.
As far as software, what I'd recommend you to do is to restore your MacBook with a fresh install of Mac OS X using the gray DVDs that came with it. Upon installing, choose as little options as possible - skip all the printer drivers, additional languages and all that stuff; most people don't need those. Run all your software updates, and keep your Mac clean using tools like CleanMyMac. I also like to run Monolingual to free some hard disk space when I install a fresh copy of OS X on my computers.
Also, you can check out this neat article at LifeHacker:
http://lifehacker.com/5252183/clean-up-and-revive-your-bloated-sluggish-mac
Hope this helps!
Ramy
Specialist
Apple Store, Carrefour Laval
+Disclaimer: Apple does not formally provide technical support on the Site. However, any information that is provided by Apple or Apple employees is offered on an “AS IS” basis without warranties of any kind.+