I'll add a bit more, I have witnessed people smash their knees too many times to not say this, NEVER underestimate the obstacle, really concentrate on getting your legs high enough, but still keep as close to the obstacle as you can.
The other point is that it is easier to do them 'over' some form of gap onto a wall, like a plant pot with slightly sunken grass or something in the middle, with solid flat topped sides. This gets the dive which you need to do one of these well. Most people forget this but you really need to dive into them, and a rail gets people scared as the hand placement is so precise. You HAVE to dive lol.
And one last thing, try to make the hand placement to be in the middle of your ark, not at one of the ends. If it's too close to the take off, it will feel too awkward and send you off balance. If you do it too late, the shock on one wrist is pretty major (think how much a saut de chat/kong can hurt if you slam your palms downwards into the obstacle.)