Forcing cleanup code in shell scripts?

[josh at offthehill.org (Josh ChaitinPollak)]

Is it possible to trap Ctrl-C and run some cleanup code in a shell  
script? How would I do that?

I've got this shell script for setting up a gentoo embedded  
environment, and it mounts a few directories, then chroots. If it  
runs successfully, it umounts when done, but if it fails, it doesn't  
run the umounts. Then I fix whatever bug it was that caused it to  
fail, and rm -rf that directory to start again from scratch. And that  
deletes my /usr/portage tree, and I get pissed and waste time.

Any ideas?