uhhh...i think it depends on the preparation. obviously avoid anything fried. sometimes they come around with the chinese broccoli or other sauteed green veggies are good. steamed dumplings?
honestly, dim sum is gonna be highly caloric. with things like this that i love where really limiting my options too strictly is gonna significantly reduce my enjoyment of it as a whole, i generally just try and consider it a special occasion. plan for it. and then just eat what i want but try and limit the super bad stuff. and then just move on. the next day is another day.
Mmm. I love dim sum also but can't have it too often (I want to eat everything). As sws mentioned, I think chines broccoli is a good option. I also love the steamed shrimp dumplings (har gau). Turnip cakes (lo bak go) are also not too bad. I also love chicken feet (but not sure if they are high in calorie or not). One problem with dim sum is the most of the items will have some kind of bun or dough around it or are fried or fatty. But take small portions of what you like and enjoy the tea (I especially love the chrysanthemum tea which is a bit more delicate--if you ask for it most dim sum places will offer it rather than jasmine tea).
It's kind of Chinese dining where they serve little dishes of dumplings, vegetables, pastries, etc. along with some tea. It's typically served in the mornings.
Most places will bring food around to your table in carts and you can decide whether you want the item or not. In other places, you order off of a menu.
I highly recommend trying it if you can find a place that serves it.