your feet should only be inserted as far as the wide part of your foot.

any further and you will break your ankle when you stack.

any less and you won't be able to control your board.

on slalom gear the back of your heals should be just touching to edge of the board. look at the strap position.

also, what type of harness do you have? what type of board?

if you are hanging of the rig correctly with your body weight then using your legs to ride the chop becomes easier and stops you from cutting through the swell. think of your legs as shock absorbers.

if your stance is to stiff then what you describe can also happen.

I think it may be your stance, you may find you are rocking onto your heels slightly even though it may not be conscious (best way is get someone to take photos). On some very narrow boards I tend to use the ball of my foot to steer the board instead of lift the toes type of thing.

Some boards it is very hard to avoid though, I had an old 8'10 slalom board of steve allen's and literally half of my foot was hanging off the side... I now have a new one of his so I will probably get this all over again . If it is the board and it is annoying take a look on the aerotech forum about 6 months back, Koen did a thing with his formula boards were he kinda built up the rail so that his foot was flatter instead of around the side of the rail (this would definately make it easier).

Personally my slalom gear I have foot in strap up to were big toe stops. Freestyle gear I have my foot further in around the wide point.

when u r new to the sport, u tend to be overly tense, and you'll find you will loosen up slowly. U tend to build muscle and burn fat really fast when in the early stages, then you get more efficient at what u do, then you start putting the fat on again Tell the missus to make the most of your first season, beautifully toned body

many slalomers will tell u horror stories of broken ankles fom putting feet too far into straps (more heresay than anything), but in reality all the pro freestylers and wavesailors recommend sticking feet all the way in till your straps virtually touch the lower shin in order to keep your weight over the centre of the board.