Try 2 beer can shims. Are you prying the stem apart at the clamp with a large screwdriver (put masking tape over it first to protect the stem)?

Use enough shims that you have to force the stem apart to get them in. The advice about the Nitto shim for the job is worth noting, but so is the fact that bars and stems each have enough variance in tolerances that success is not guaranteed. (I suspect the Nitto products will be pretty close, but other manufacturers not so much. My framebuilder won't make me a shim until he has actually measured both parts.)

Ben