Its so much easier to do tricks off of grinds if you compress your knees SLOWLY while you're grinding then pop STRAIGHT off what you're grinding. So if it's a flatbar, pop straight up, and if its a down rail, pop straight out, perpendicular to the rail.
That's the biggest mistake people make, either not jumping off squarely, or trying to compress and hop in an instant, which results in a loss of balance.