"How-Tos" for Rolling Your Own Short URLs?
mdakin: I would associate each URL with a sequential number.  And express the number using base-36 (0...9 + a...z).  I don't see the need to use obfuscation for this application but if desired I'd hash the URL + a secret and prepend that fixed-length hash to the base-36 sequential number.  I'd express the hash in base-36 too.