Avoid the boss compressor/sustainer, you can get much better compressors for that price (Alesis 3630 for example). Youll get a maximum of 1 second more of sustain from it, like you would with most compressors, and it will colour your tone horribly.
First thing I would check is your strings. How old are they? Old strings (depending how long you play, but you should change them at least every 2 month, even if you only play once a week) wont sustain as much as new ones. Try popping a new set on, it could improve the sustain heaps.
What guitar do have? Certain guitars wont sustain as long as other ones, and it would be helpful to know if youve got one of them.
The other thing you could do is... TURN IT UP. Besides being more rock and roll, turning your amp up will give you more sustain.