I keep getting flats right at the valve!
As Paparazzi said: make sure the valve goes straight through the valve hole, i.e. perpendicular to the tube. This is likely the solution, but I've seen it happen that even after properly aligning the valve it somehow wasn't aligned anymore after a couple of rides. Which resulted in the bottom of the valve getting cut by the edge of the valve hole in the rim. Which means the tube did not (or not completely) stay in place, i.e. at least a part of it shifted with respect to the rim (and possibly the tyre, hard to tell). As far as I know two things could cause this, first one more likely than the other:


if your tube diameter is bigger than your rim and you force it in the tyre anyway, there whill be one or more wrinkles in the tube. If this wrinkle is near the valve, it's not too hard to see that it's possible that under stress the tyre can move a bit, resulting in an unaligned valve, resulting in cutting
even with a proper tube diameter, if your tube sticks to the tyre around the location of the valve (caused e.g. by lack of some antistick agent like chalk, water getting in, ...) and there is quite a lot of stress on the tyre, the tyre could in theory move with respect to the rim and drag the tube with it. Can, as far as I know, only be caused by skidding or incorrectly landing jumps or so: unless the tyre is the wrong size, it requires sheer force to rotate it over the rim.