It sounds like you might be too worried about gaining fat (takes one to know one [img]smile.gif[/img] ).
The simple answer is to eat more. The complex answer is to eat more of the right foods at the right times, in quantities that are sufficiently above maintenance to be utilized by your muscles, without excessive surplus to be shuttled into fat tissue. How is this done? It depends [img]tongue.gif[/img]
Here are some places to start:
Massive Eating Reloaded
Massive Eating Calculator