With difficulty! Our New Forest mare ended up looking like she was having triplets and she was kept on a small bareish paddock during the day and in at night..
All I found we could do was the usual things with fatties - small paddock without much grass, double netted haynet, not too much hard feed (but ensure sufficient nutrients for her and the growing foal) etc.
Good luck and let me know if you find any other ideas!
There are some great feeds out there now, whereby you only have to give a very small amount each day to give the mare the required amounts of nutrition. Soaked hay, and plenty of it, will give her enough fibre and ensure that the gut is working well. Exercise is always the best form of keeping the mare fit and healthy though, even if you are just taking her out for long walks (if she can't be ridden). Alternatively, if you have a field with long old grass, this is ideal for a non-ridden broodmare.