We always wean 'cold turkey'. By the time weaning comes around (approx 6 months of age here) the mares and foals are back in with the rest of the girl herd and have been for several months. They are all living out 24/7 with free range of several fields of good grass, foals are often found in their own group in a neighbouring field to their dams - more or less having weaned themselves.
We bring the mares and foals up to our 'weaning' field - it has a big run-in barn - and simply pop the foals in through the gate, wait for them to speed off to investigate their new surroundings, then lead the mares away and return them to the herd pastures. The mares never look back, and the foals seem unconcerned. Never had any problem doing things this way. Like Diane I certainly wouldn't restrict any mares food (or grass in our case) and never her water, we just keep an eye on any udder development, but mostly if they do fill, the lack of suckling soon stops the milk production without a problem. I do think that the earlier you wean the more problem you will have with possible udder development, plus I also think that the fun time with foals starts AFTER weaning - no interferring Mommas wanting to know what you are doing with their babies. LOL!!