Originally Posted by JohnDThompson

Yes, clamp on the seatpost, unless you have a carbon fiber seatpost. If so, get a cheap aluminum or steel post of the proper diameter, remove the carbon post, insert the metal post and clamp on that. Or, if you want to get fancy (or just have multiple bikes with different seat post diameters), get the Park "Internal Seat Clamp" tool:


ISC-1 Internal Seat Tube Clamp | Park Tool