I had an older Compaq Presario that I cracked open (Cyrix 233 in that thing and it was one HOT runner!

) The side of the case had an itsy-bitsy exhaust fan that I blew the dust out of, and cleaned off the CPU heat sink the same way, with a can of compressed air. Other than that, it was pretty clean in there.
A lot of the heat buildup, under normal conditions, is on the bottom of the case. There are some aluminum panels you can get now, kind of like the top of a lectern, that have fans in them. They serve a dual purpose of getting the bottom of the laptop off of a flat surface with no ventilation, as well as blowing some air across the bottom to help heat dissipation.