This is the only trick that I feel I'm really good and can make look great. It's probably my best quality as a person. If I happen to have an off day with them I feel like my life is falling apart.
My back foot is centred at the end of the tail and it's pretty much perpendicular to the board, but sometimes angled every-so-slightly forward. My front foot is at about a 30-degree angle forward and it's actually a few inches behind the bolts. I see a lot of people with their front foot much higher up on the board, but I find I get less height that way. If I'm just doing one on flat, my shoulders are pretty square, but if I'm going down or over something, my shoulders more closely approximate the angle of my front foot. I don't concentrate too hard on popping really hard and forcing it; the more relaxed I am the better they feel.
This is exactly how I feel. Kickflips are like my go to trick; probably the only one I really have on lock every time and I'm really good at. I do them on flat, over stuff, down stairs/gaps, off of banks/hips, even on tranny. I had a terrible time learning heelflips though and I'm still not great at them.
One thing that I've learned is that the further that my front foot is on the board the higher/cleaner/better my kickflips are. When I say "the further my front foot is on the board", I'm talking toe to heel ratio. I basically keep it right under my front bolts and have most of it on the board, I'd say 3/4 of it with only a bit of my heel hanging off. A lot of people think that if they have most of their foot hanging off of the board and just use their toe to flip the board it'll work better but that's not the case. You just need to pop and flick at the right point and it'll come right up to your back foot after flipping.