[I see that you have tagged this with the thickness-planer tag, so this answer probably does not apply because it refers to hand-planing. I'm leaving it here for posterity I guess.]
The one technique I'm aware of (and have actually used!) that does not require special tools or jigs is to hand plane the board along its length in quarters.
That is to say, you apply some amount of strokes about a quarter of the way up the board, all the way across the width. Then do the same, but plane 1/2 the length. Then 3/4 of the length, then finally the whole board.
This results in a wedge profile that you can tune appropriately. I suppose one could tune the "quarters" to some other value to get a different profile.