Never shift while stationary, except when going from neutral to first. Never shift with the engine not running, unless you have to. To go from first to neutral, gently roll the bike a couple of inches while simultaneously pulling up gently on the shifter. Once you get the hang of it, you'll see how easy it is.
Make sure you're clutch is in good shape and adjusted properly, and you're using good quality oil.
If you find yourself in, let's say, 3rd gear with the engine off, after a tip over or something. Pull in the clutch, gently roll the bike back and forth, and gently click down one gear at a time to first. Then back to neutral as described above, and restart the engine.