Quote Originally Posted by Popoki_Nui View Post
You can also wrap some medium sandpaper (100 grit) around the seat post and gently roughen it around its diameter (i.e. not lengthwise) where it fits into the seat tube. This will give it a little extra grip.
It's an obvious thing, but I'll say it anyway: Before you try this, make sure you don't have a carbon-fiber seatpost!