I'd recommend keeping them in a closet.  Wherever 
you end up putting them though, you should try to 
put them on or near the ground (if you're worried 
about the heat).  In some places more than 
others, this makes a huge difference in 
temperature because of the fact that hot air 
rises.  The cool air will settle to the ground 
and that's where your cels should be ;)

I keep my cels on the ground under my bed, and 
even when my room heats up to about 85 degrees, I 
can pull out my cels and they're still at a 
confortable temperature (more like 75 degrees).  
This kind of thing depends a lot on your 
house/condo/whatever--just try searching for a 
place in your house that doesn't get any sun and 
stays relatively cool.

-Graham