Paul Burke mentioned cliches and I think this is part of it. Identify a part of the tune that is similar to something you have heard before and play it. Add another section and so on. If you are playing along, say in a session or to a tape, try to pick out the notes on the strongest beats and the next time througn the tune try to add some more. I think it's a lot easier if you learn to hum the tune first but you won't have time for that in a session.