By using percent values for bottom, left, right or top in conjunction with position: relative;.
I use top and left, I've had varying results with left & bottom.
This puts the top left corner of <div class="somediv"> to 5% of the top left corner of the parent container.
Other properties such as padding, margin, width etc.. can also come into play depending on your layout.
There's some good info & links on layout techniques at glish, indexdotcss gets into the specifics pretty well.