Ch'eng Man Ch'ing writes in his Thirteen Chapters that the weight of the Body should rest on just one foot.
Otherwise you are double weighted (Although I think that double weightedness is also a state of mind).
But when practising my form I watch for tension and where it is being held, trying to let it dissolve so my energy can sink down to my feet.
When I first started learning Tai Chi one of the more advanced students also told me to relax the soles of the feet so there was no resistance to the ground.
How do you work on this?