We were giving our daughter meal replacement shakes a couple times a day as a snack. That worked so well that the nutritionist bumped her down to one a day.
We also put salted butter, cheese and/or ranch dressing on everything. She loves fruit, so I sprinkle it with cinnamon and sugar.
It's hard somethimes to maximize calories when everyone else is trying to reduce them. Not too long ago at a Smoothie place I asked for a little weight gain to be put into Reagans smoothie and they wouldn't do it. I explained that she had special nutritional needs, but still a no. I understand their reasoning behind it, but surely they could see that my 17 month old was no weightlifter in training. lol.