Again, I must tip my hat to the one and only Madame Hodgkins for this invaluable tip:

When making salad, always mix up the dressing in the bottom of the same salad bowl you’re going to serve it from, then throw in the tomatoes and any other chopped vegetables. I like a little avocado and red onion, but if you feel the need to bung some carrots or cukes in there, have at it. The last thing to go in is the lettuce. (And really, does anyone use anything but the pre-washed, bagged stuff anymore? I know you’re supposed to wash it and I always intend to, but…Anyways, you really should wash it before you eat it. Let’s leave it at that. But I digress.) The only things you really should hold off adding until the moment of service are crumbled cheese and nuts.

So once you’ve got the dressing made (unless of course you’re drawing from the big stash in your fridge) and everything in on top, you can cover it with a damp paper towel and let it sit on the counter for a really long time — I’ve let it go two hours — with no ill effects, as long as you don’t toss it.  Leave that until the moment you’re ready to serve it.  Takes so much stress out of company meals and allows you to spend the time you’d normally be assembling salad having a life giving drink and a happy chat with your guests. Perfect!