How, and how often should I clean a freezer?
I don't usually bother letting it warm up.  I just use a mixture of a mild kitchen cleaner and warm water on a paper towel.  If you're careful, and don't let any moisture pool up, it seems to evaporate before it freezes to the inside.

If you're just trying to combat a smell, try putting some baking soda in there.

As for how often to clean, I usually do a purge of the contents of the freeze 1-2 times a year, and do the cleaning then.