One of GarageBand’s useful little features is something called Master Tempo. Master Tempo lets you easily change the temp of your recording with a few click-and-drag actions of your mouse. To work with Master Tempo in GarageBand, select Track -> Show Master Track (or just press Command-B). In the new section that appears in the Tracks portion of the screen, set the pop-up menu to Master Tempo.
To change the tempo of your recording at any point, just click somewhere along the timeline. This will create a new handle (dot) that you can then drag up or down to change the tempo. If you just drag it, though, you’ll see that it jumps in roughly five to six beat-per-minute steps. But what if you don’t want to make that much of a change?
For more fine-grained control, after creating a new handle, try holding the Shift key down before dragging it. This will move the slider in one beat-per-minute increments. This trick also works for the Master Volume section, which usually moves in 0.5dB to 0.7dB steps. Hold down the Shift key, though, and you can make changes in the range of 0.2dB to 0.3dB. (Master Pitch already moves at the finest level possible without using the Shift key, so nothing changes when you hold down Shift and change the Master Pitch.)
Thanks to Mac OS X Hints reader asmeurer, who points out that this trick also works with some of the track automations, such as frequency, though not with all of them.