I've been diagnosed Type one for 6 months and have actually found it tricky to put on weight. I lost 2 stone before I was diagnosed back in June and I'm still 1/2 a stone off my previous, healthy weight.
I'm guessing this is because I've cut out the sugars I used to eat in things like biscuits, sweets, chocolate, cakes etc (bar the odd slip!) I guess that old nugget of eating healthily and trying to get exercise (easy said then done when a Type 1 diabetic) is just as applicable, if not more so, if you're looking to keep weight off / lose weight.
But hey, I'm only 6 months in, who knows what might happen over Christmas!