I imagine you're asking because you're getting some trips when the compressor and heater kick in at the same time. You have 15 amp gear and 15 amp receptacles and you're thinking bump up to a 20 amp circuit for the receptacles to prevent those trips. That might work fine.
If it isn't too much trouble, you could put a receptacle for your compressor on one circuit and a receptacle for your heater on another circuit, on adjacent spaces so they're not on the same leg. Then run the lights and remaining receptacles on another circuit. Might not be necessary but balancing the loads out might make for less trips.
If you want to go even further you could split your lights and other receptacles on two separate circuits on different legs, so they are balanced out too.
edit: Tester101 and others mention that your feed is undersized if it's only 12 gauge, that needs to be checked and addressed. If you have to use a smaller main breaker on the subpanel, balancing out your loads may be more important, but there just might not be enough juice to do what you want to do.