have you been using the same yeast every time? if so, try a more attenuative strain. for high og beers, you need to pitch lots of yeast cells, and aerate the wort really good. using a yeast nutrient can help too. I'm guessing you are doing extract brews, and they are famous for stopping about 1.020. you could try a different brand of extract, as some are more fermentable than others. sometimes adding a pound of honey or table sugar can get the fg to go lower, but don't use too much sugar. it'll thin the beer out as well as dry it.