Does the shifter move a lot? Could it be your linkage is loose?

I'm with Josh, though... it sounds like it's your clutch adjustment. Instead of mashing on the shifter to push it into gear, press the shifter lightly and see if after a while, it goes in on its own. The harder you mash on the shifter, the more friction you're asserting on the synchros... that's why they go bad.

Often times when the clutch isn't disengaging fully, you'll feel that if you're trying to speed-shift because the engine rpms have to drop low enough for the next gear. If this is what yours does with light pressure on the shifter, go get your clutch adjusted.