For towels etc, we drape them over the bullbar / UHF aerial (ours is the solid fibreglass type). Smaller items get pegged to the cargo barrier. If we really need to, we string up some rope, which we always keep in the 4WD along with some clothes pegs.
We also have one of those collapsable "Hills Hoist" things but we've only taken it away once. It works well enough, however we find it a bit bulky and it needs pegging down or the wind tips it over, so it stays at home where it actually gets used all the time.