If you're not approaching the amperage limit of the box, but rather just have a lot of circuits on there, you can utilize the dual switch single space breakers to free up some space. For instance if you have 4x 15amp circuits all in a line, you can get 2x dual 15amp single space breakers, and then you'll have space for a 240v breaker. Thats what i did when we put in our hot tub. Our house is weirdly set up and we have a lot of different 15amp circuits that feed like 2 or 3 outlets tops in areas that dont get use.