I wash them after every wearing.
My 6 year old Jockey For Her boxer briefs are still going strong. They go through the dryer, too. (the rest hang to dry)
My 2 year old UnderArmors are pretty much like new, except where the pen I carry in my pocket at work has roughed up a patch on the left thighs.
My 2 year old New Balance "brushed" wicking fabric are covered in little pills, but are still doing fine. They accidentally went through the dryer a couple times.
My 1 year old Ibex are like new, and I wear them a *lot* and wash after every wearing.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson