Some people may find these easier on flat than kickflips but to 99% of skateboarders find they are definetly harder down drops and over gaps. I suggest for Heelflips that you put less of your foot on the tail and put your front foot in the middle of the board with your toes hanging off the edge of your board. Some people tend to try heelflips with there foot too much on their board and angled towards the nose a bit like a kickflip, i suggest against doing that, keep your front foot straight.
If you have already got a decent ollie you tend to automatically slide your foot up the board before kicking it off the nose, this helps the heelflips come higher but may be a little harder to land. First of all when you bend down lean your weight a little behind you, I find this helps to keep the board underneath you properly, and pop as you would for an ollie. With your front foot try not the slide it up too much and kick it in front of you. This should get the board to flip, jump higher than you would for a kickflip to let the board flip and catch the board with your feet. You have to make sure your front foot kicks out and comes back in to meet the board quickly.