I got told to damp the sheath with water and get it nice and soft, then wrap your blade in a couple of layers of clingfilm, put it in your sheath and use some clamps and something to prevent marking the leather. Tighten up the clamps enough to get the fit you want and put it all somewhere to dry for a day or two.
I`ve done this to tighten up a sheath after it`s become loose after a while but I`m sure some of the leathercrafty folk could tell you better.