Spin, spin, spin city. As you can see, my lettuce spinner is fairly small, so I had to do this in two batches. No worries, though. I actually prefer it to having a giant one take up valuable storage space. Anyways, now your lettuce is dry- as dry as you can possibly get it. Good. Line your storage container with your dish towel and then add your lettuce. Cover the lettuce with the dish towel, then snap on your lid. Your lettuce should stay fresh for a week. If you change your dish towel daily it could last up to two- but if all goes as planned, you'll have eaten it long before that, right?
As for adding other veggies to the lettuce, I wouldn't. Especially onions- the acid will definitely make your lettuce go bad sooner. But I do keep cut onions, bell peppers, carrots, and whatever else I'm into at the moment in the fridge ready to be tossed with my lettuce. Well, folks, I'm all out of space. Until next time.
Update: Also works fantastically well for kale. Those bunches at the grocery store are always sopping wet and bound to go bad quickly. Remove the thick, woody stems and give them the same treatment, preferably as soon as you bring them home.