Mostly I just let the stank build because I'm in the backcountry and either I am alone or with other stinky people.

I do have a small container of biodegradable soap with me and I'll clean my underwear after 2-3days to ensure I don't get any saddle sores. I don't wear padded shorts so I just wear Exofficio boxer briefs which dry fast.

If the tour is long enough or muddy enough I'll do laundry more frequently. Usually near a creek with the waste water dumped far enough away to biodegrade as it works its way back into the water table.