Stairs and upholstery are 2 of the services a lot of carpet cleaners do not like as well as carpet cleaning.
Having done this for 23 years now, I believe I've got this stair cleaning thing going well for me.
I use the PMF swivel stair tool (which is about 30" long), long enough so you don't have to bend over too much and kill your back. Instead of cleaning in the forward and back again motion (Backgammon pattern), as you may do when cleaning carpet, I start at the left edge, with the head of the tool parallel with the wall, not perpendicular. This allows me to edge clean where the carpet meets at the tread and riser (one of the crevices where hair and dust collect) in a side-to-side motion, with the tool ending up tilted out over the nose (side-to-side still) of the tread, so the tool can blast straight down with the 06 jet on the nose (which is also usually very soiled).
I do three treads at a time before backing down for the next 3 (including the turning of the tool head for the 3 risers), and then go back and do the same motion (side-to-side) as a vac stroke only, then take off the hose end and vacuum these 3 tread noses.
I've tried having the vac and solution hose attached to my belt and found it uncomfortable, so instead, I pull the vac and sol hose a little longer than it needs to be, and bring it up behind me on the left side, and loop it back down my right side about 4 feet (ankles now looped once by the hoses) so the hoses won't be fighting you wanting to go down the stairs while you are cleaning. With this procedure, the "tug" of the hoses is at your feet and is not interfering with your cleaning.
This is quick with plenty of flushing action for cleaning, with a good final vac, and easy on the body.