Foot Position on Pedals
As for foot placement, it's no different for jumping than it is for any other riding. You want the ball of your foot over the spindle of the pedal, maybe slightly forward (5 millimeters or so) for flats. Feet straight, or even just a tiny tiny bit toes in, or heels out, to keep your knees and hips aligned and working correctly.
Slipping a pedal on a jump is all about technique in how you load the bike during as you push into the face of the jump and how you unload as you leave the lip, and what your heels are doing during each part of that action.
Heels down as you press into the jump, toes down as you take off. But here's the hitch: You don't want to lift or huck the bike into the air. You want you and your bike to travel the arc of the jump together, and you need to be loose and relaxed, with your hips over your bottom-bracket, not way back over the rear hub. When you pull and force the bike up into the air, you tighten your body and it no longer is working with the bike. When you're tight, the bike is in control, but it's in bad control. When you're tight, the bike will kick and this is where your feet separate from the pedals.
You can always feel that tightness in your elbows and shoulders. When your elbows are bent and all is relaxed, you can feel control, and that applies to everything you do on the bike.
Rich Drew is one of the best coaches in the game and has several excellent videos explaining the process. Tim MacCormack is another great coach with videos out there. Check those out and go back to basics and start small again.