If you have an older XBox 360 that has over heating problems (you can tell because the system freezes up when it gets too hit), there are two things that have worked for me.

First, I've got one of those laptop coolers that you can get at any computer store for around $20 and I put my XBox 360 on that. It helps quite a bit.

Second, gets yourself a small fan and point it at the side of your XBox 360 with the vent holes. That'll help bring a continuous stream of cooler air into the XBox 360 to cool off the components.

Since I started using this two tricks together, my XBox 360 has never had any over heating issues.