A friend of mine had an older 2008 MacBook Pro (the kind that does not have the modern 'unibody' construction), and he noticed it was getting slower. He upgraded the RAM to max it out at 4 GB (I think it might be able to go to 6 or 8 GB if needed). But a lot of things took a long time to do, even though the Mac had a 1.86 Ghz Core 2 Duo processor (not a slouch by any means).
He asked me to replace the hard drive with an SSD, so I did. I followed this iFixIt guide, and put in a new OCZ Agility 256GB SSD, which is way faster (especially for random access, like when you boot the computer or launch an app) than the old disk drive that I removed from the MacBook Pro.