I feel you here...
I have had to train a ten year old Belgian cross mare to pick up her feet. Her previous owners had just forcefully tied her. Very big feet....
More recently, my 4 year old, Friesian cross, 16.1 hand gelding has had issues as we just purchased him about 6 months ago now.
I personally start with teaching them the verbal cue. Either "up" or foot
When they even switch their weight off the foot, you give reward, either cookies or scratches. You then progress to actually holding the foot for longer periods of time.
My 4 year old gelding got into the lovely habit of trying to sit on the farrierwhen he did his back feet. He would just generally be rude. So I would hold his feet for increasing increments of time. When he would try to sit on me I would smack his belly with the lead rope or lightly with a whip.
It is best to start with small increments so you can place your horse's foot down, not have them take it. This way they get the reward and understand the concept. However, if they refuse to progressively hold their foot for longer and attempt to pull, belly smack comes in and no cookie. In all these attempts you use the verbal cue as well.
This is a slow process, but I find this is the best way to avoid conflict and make them understand it. I would pick up Tungsten's feet maybe 4-5 times a day. Groom a few minutes, pick up feet, comb mane, pick up feet, ect.
The above method works as well, but my guy is not comfortable being tied by his feet, and it can lead to more fighting.
Anyway, if you have more questions or run into issues, you can PM me. I have experienced almost every trick in the book when it comes to picking up feet :) Good luck