It's important to make sure that there's a sign on top of the signpost you choose to lock your bike to. Here's what you do if you're locking up outside a bar and you realize your friend has locked up his bike to a sign with no post. You lock yours someplace nearby, and head inside. Have a few drinks, and once your friend gets up to use the restroom, take a barstool outside and stand on top of it next to his bike so that you can lift his locked bike up over the signpost. Then I usually like to lock his bike to mine in a different location so that he thinks both our bikes have been stolen when we later walk out of the bar together.