I buy the frozen daphnia that comes in cubes. I tend to feed it once a week to one every other week. I haven't had a problem with constipation yet. I just make sure to have it on hand just in case.
I have also been feeding pellets less and less. It really depends on what you are comfortable with and the individual fish. When we first got Hannibal, all he would eat was freeze dried blood worms. Now he eats anything I give him, but it was a slow process. Each of my bettas has their own tastes:
Uther prefers mosquito larvae. They are his favorites. I don't argue with him. If I have leftovers from feeding the rest of them, he usually gets more the following day.
Sholto favors brine shrimp.
Conchenn is my little piggie. She will eat anything and everything. She really loves blood worms and live ants though. We have sugar ants and I learned when one fell in the tank she loves them. So now I will get them from time to time and feed them to her and a couple of the others.
Odin only really like pellets and mosquito larvae. He has sight problems.