I rack my single-length runners on my harness (tripled of course), but I put my double-length runners around my shoulder. I clip the opposite end of the sling with the biner (like a basket hitch) and then put it around my shoulder. I learned this trick from a Steve House video a couple years ago (skip to 8:00 if you want) and it's a very easy one-handed method of getting to your double-length runners.
I put half as alpine draws on my back right gear loop, and half with one binder around my shoulder (right shoulder to left hip) that way, if my left hand is occupied and I can't free the shoulder slings, I can use my right hand to get some off my harness.
No one has brought up the point yet that if you carry slings around your shoulder and you can't free up that hand you can still clip the piece of gear and the rope, you'll just get your hand out as you climb past.
I've been carrying my 24" slings over the shoulder with one biner each and haven't had any real problems (although admittedly I don't lead at my full limit most of the time).
Any 48" slings I'm carrying (also with a single biner and basket-hitch style) go underneath the 24's and in the opposite direction since it's easy to unclip and slide it out.
I only wear 4' slings over my shoulder with one biner connecting the ends. The advantage is that you don't have a long ass double length hanging from your harness. You simply unclip the biner from one end of the sling and it slides right off. No having to pull it over your head and arm nonsense.