I like these by Amco Houseworks (they make one each for oranges, lemons, and limes). The center section gives you additional leverage. I also a) cut the ends of the fruit, to give a flatter surface and better/more even pressure on the rind, and b) score an "X" pretty deeply on the cut half of the citrus in question. Tons of juice doing things this way.
If a particular fruit is too big for the big (i.e., "orange") juicer, like, say, a grapefruit, I just cut it into a quarter wedge and line it up so that the long way of the wedge is running along the same line of the handle, then I just juice away.
When the time comes to juice for a brigade of people, I grab one of those countertop lever models.