MY experiences
1. Downgrade your Operating System to have more free RAM
System Requirements for OS:
Lion 2 GB
Snow Leopard 1 GB
Leopard 512 MB
..256 MB
2. Replace RAM
Its not just the size important, age too +....
ifixit.com free repair tutorial, in 5minutes you have replaced your ram
3. Replace Hard Drive
Are expensive now, because of Thailand.
4. Use only 1 Disk Partition
5. Open your MB and clean it inside. Hairballs..
6. Do not update after a clean install of Mac Os x. After updates they run slower.
7. Do not use Onyx or Monolingual, TotalFinder, LittleSnitch Tools, new Firefox versions. New Firefox versions needs for example more RAM (System Requirements Firefox).
8. Look at the Activity Monitor for CPU and RAM usage.
9. Disable Applications on Start Up, Login Items.
(10. disable Dashbord, Spotlight with Terminal Code)
11. Set Brightness of Monitor to low. (heat)
12. replace iphoto with xee, itunes with vlc and so far. use applications and versions, which arrives before your Snow Leopard.