Like a lot of people, I'm enamored of Apple's GarageBand and have been spending more time dinking around with it than I should. As I do that dinking and stumble across helpful tidbits, I'll pass them along in this space. Here's tidbit number one:
When you record a track with Apple's software instruments (IOW, use a MIDI controller to record MIDI data into GarageBand) and record that track with looping switched on, the program will continue to record what you're playing when you return to the beginning of the loop -- layering it on top of what you've already played. Although the unwary may inadvertently pack more notes into a track than is considered tasteful in polite society, this can be helpful in a couple of situations.
Situation 1 is when you're recording a "live" drum track with one of the included drum kits. Use the first pass to lay down the bass drum. On the next pass you can add the snare. On the third pass add hi-hat. On the final pass add cymbals. This one-drum-at-a-time technique allows you to concentrate on smacking the key at the right time rather than worrying about finding the key that triggers the closed hi-hat sound.
Situation 2 is when you want to add note bends and modulation to a track. GarageBand doesn't accept a lot of different kinds of controller data input (I doubt a breath controller would have any effect, for example) but it will pay attention to a MIDI controller's pitch and modulation controls. To avoid the distraction of bending notes or adding modulation while you're playing a lead, play the lead, wait for the track to loop, and then wiggle the pitch and modulation wheels. Those wiggles will be recorded into the track just as if you'd played notes with your controller.