I have a basic home studio setup with a Macbook, Apogee Duet, Logic, and a few microphones. Most times while recording, the fan on my laptop is spinning furiously; this is a problem when I record acoustic/classical guitars. How can I quiet my noisy laptop?!
Put it in a different room!
Professional studios do this by having a control room with all of the more noisy equipment, so in a home studio you're going to have to try to approximate that environment as best you can.
On a MacBook, you may be able to limit CPU usage in the Energy Saver preference pane. I can't tell you specifically since I'm on an iMac, but I believe you can elect to constrain processor activity so it doesn't generate as much heat (and thus, require that the fan spin up to 100%). Of course, a more powerful computer would also improve the performance/heat output ratio.
To have lower fan speeds while also not having a cooked laptop, you need to have less heat to remove (i.e. reduce CPU or GPU load) or more efficient passive heat transfer. Luckily, modern MacBooks have metal cases which are quite thermally conductive, which allows many possibilities for improving cooling.
Chill the air around the laptop as much as you can (air conditioning, record in the winter, etc). The colder the environment is than the processors, the faster heat transfers out, so the less fan assistance needed.
Use larger, quieter external fans to move air past the laptop.
Place the laptop on a metal or glass table, for better conduction of heat transferred from the underside of the laptop.
(You can also attempt to dampen the fan noise or isolate it from the recording; I'm addressing only improving cooling in this answer.)