I've wondered this my self a few times.
But then again, i use panniers so i never run into this unless i have both filled and something large strapped across the back.
When i do have something large on the back (depending on the weight) I swing my leg way over the saddle, over the handle bars, or over the top tube.
And to the OP: I recommend on just picking up the cheapest panniers that could work for you when they go on sale at nashbar. If you move the weight that your hauling farther down and balanced it is a much more pleasurable experience.