Saddle keeps slipping

Despite having the seatpost clamp bolted up tight, my saddle continually slips down - and I had to stop 3 times today to sort it out over 20miles as I'm losing a cm or so every 6 miles it would seem! Any suggestions? Do I need a new clamp? Or is there another fix??
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the hairspray option works for me...use sparingly as I got b;;ked for using it when it cost £8 a can
apparently... :?