Basically riders lose their strirrups because the leathers are too long,, the foot is not against the side of the stirrup iron (inside for jumping/outside for dressage riders), or they are not open from hip to heel (more parallel) and need more stretch from hip to heel.  The heels must be lower than the toes, not by stressed action, but 'elastically' lowered.  If the thighs are too high there is no stretch into heel, no sustaining the foot in the iron (another problematic result of exceeding padded saddles with massive thigh blocks which not allow the rider to find their seat/balance properly).

The best way to do this is spend time (LOTS of time) in two point, to have the stirrups shorter, keeping the leathers vertical, keeping the hip open, the thigh lowered, the knee lowered, all w/o pushing.  IF you can do this is w/t/c, then you can do the same thing seated.  It will NOT work if you turn your toes out.  Also, the use of irons with 90 offset eye ( http://www.doversaddlery.com/herm-sprenger-offset-eye-stirrup-irons/p/X1-0708/cn/97/ ) will help the leather be flat on the saddle, and keep the iron straight across the foot.

You MIGHT be unconsciously gripping with your knees.  This effectively shortens the leg, leading to floating stirrups, this has been long a problem for me!

For me ANY strong grip with my legs, knee or upper calf (though I do better w/ upper calf) leads to my stirrups sliding home.  I noticed this just yesterday during my lesson.  Definitely shorten your stirrups, this is just admitting reality--gripping leads to a shorter leg.

You might also want to think about relaxing your leg after giving a leg aid so your leg will stretch out.