One of the best ways is to feed the supplement to your crickets and mealworms. You can buy high-calcium cricket food or you can mix a little supplement with baby cereal and feed to the insects for a couple of nights before they are eaten. Supplements should be given no more than 1-2 times a week. The main vitamins and minerals you need to supplement are calcium, vitamin D3, and vitamin E. Fruit flies tend to be a problem when you leave food out too long. Since you shouldn't use any poisons to kill them, getting rid of them can be difficult.
Several things help to keep their numbers low.
First, take out uneaten food as soon as possible. Also, you can clean their cages every week. The best way I have found so far is to pour a little dark cider vinegar, red wine, or beer into the bottom of a jar. Then cover it with plastic wrap, securing with a rubber band and poke 6 - 10 fruit fly sized holes in it. The fruit flies climb in but can't get back out.