Use an old chain and a trashed innertube to keep your seat by getting stolen. Most bike related theft is prevented if the act will take the theif longer than a few seconds. Even if they do have a bolt cutter on them, they're less likely to take your seat if they have to pull the bolt cutter out of their bag.
We learned this trick fromStreet Level Cycles
at Aquatic Park in Berkeley. Support your local community bike program!