Windsurfing & Stand-Up Paddleboarding in Michigan
My go-to technique is to attach 2 booms to the mast and counter-rotate them. If that doesn't do the trick, get a couple friends to help. Have one person repeatedly flex the mast at the joint while the others counter-rotate the booms. If you don't have two booms, then flexing may be your best bet.
If Larry's suggestion doesn't work (it should), then I have a water-pressure method that is guaranteed but requires some fittings and a hose.