How to keep your seat on your bike (a deterent)

1. Get an old bike tube.
2. Get some old chain.
3. Measure a length of chain that is long enough to loop through your seat rails and seat stays of your frame.
4. Use a chainbreaker tool to separate the chain at the appropriate length.
5. Do the same with a length of the old tube. Use enough to cover the chain fully as to not have it wear against your frame (if you care).
6. Lower your seat slightly to give you some slack when you reconnect the ends of the chain using your chain tool.
7. Raise your seat back to the appropriate height and Voila!

This is a fairly effective and virtually cost-free way to deter seat/seatpost theft. It will also get a lot of questions from people who can't logically deduce its purpose. Have patience with them and drop some knowledge.

Here are some pics stolen from HKFIXED