Step one look at the seat tube slot when the seatpost is clamped. With a properly sized post the slot will be pinched closed slightly at the top. With an undersized post there will be more pinching, and if the post is much too small the top will be closed on itself and the post cannot clamped tighter no matter how hard you try.

If the slot closes to touching or nearly so, buy the right size post. Period

If the slot still is 1/16" wide at the top, a better collar or QR unit might do the trick, though your best bet is to paint the 2" section of the post which ends up under the clamp with something that will improve traction.

Carbon assembly paste will work, but something less expensive like latex paint might also. Before painting be sure to strip the post to bare metal with acetone (not nail polish remover), naphtha, or mineral spirits which will leave it clean, dry, grease free and ready for the coating to stick.