An additional note to the top answer:
Since you can either choose to disable the sleepimage, or activate it but then be forced to keep it after wake up, i found a dirty little trick for my preferred solution: Have a sleepimage but delete it after wakeup to free the disk space. If you are someone like me who constantly hovers at around 2-6 GB free space on your small SSD, this might be your solution.
Just point the sleepimage to your
.Trash folder and empty your trash bin after wake up.
sudo pmset -a hibernatefile ~/.Trash/sleepimage
This is only a viable solution, if you only use one account on your mac. You wont be able to easily delete the sleepfile from another account (Of course you could if the other account has sufficient rights, but it would loose it's simplicity because it would not just mean clicking on the trash bin).
If for whatever reason you want to return to the factory settings, just type
sudo pmset -a hibernatefile /var/vm/sleepimage
(Note: I use 10.8, this might change in other versions, just check
man pmset to be sure.)