As long as the set up on your Les Paul is correct, it should have alot of sustain itself. You can add to what it does have by using heavier strings, and using a compressor, both things said above.
Using more overdrive will add sustain, but it will mush up your sound. Simply turning the amp up so that the sound from the amp interacts with your guitar will add quite alot of sustain that is good once you learn how to use it.
H2H