off the bike do this:
put your feet about where they'd be if on the pedals with the cranks horizontal
now hop forward a few inches using nothing but ankle

now get on the bike, use the same type motion to unweight the rear of the bike, it doesn't have to be very big, its actually a very subtle but sharp and short movement........someone watching might not even see it, its all in the ankle or ankles, you only need to get your weight off the bike for a split second

by getting your weight off the bike and causing the bike to get unweighted too you won't even have to worry about putting all your weight as far forward as possible, the rear will lock up quite easily when it isn't fighting your body weight

this can be done in the saddle and done repeatedly by rocking the cranks doing a whole bunch of little skids, its like pumping the brakes in a car

unweight and pull up with front foot while pushing down with rear, for longer skids depending on bike layout and your strength yes you may have to lean forward quite a bit, but for short skids which slow you down quite quickly you won't have to lean forward at all

and once you get good at it, crank position won't hardly even matter, you will just do it without even thinking about it