Use a BRAND NEW sharp Xacto blade or single-edged razor blade. Always use fresh blades, don't use old ones. You want them very sharp, if they're at all dull, you'll end up crushing and snapping your way through the tiny parts. This may seem wasteful, but it's the way to get results. If you have two or more X-acto handles, always put a fresh blade on one, and use the other for less precise jobs. Whenever you replace a blade, always replace the older one, and then both your blades will always be really sharp, although the older one may lose the point of the blade if you're scribing or cutting plastic sheet. That fine point is easy to break off.
Also, make very light passes, don't apply pressure to the sprue gate.