If you don't have tables for everything, take a long shelf from inside your house and put it across two larger boxes. Even lining up boxes that are similar in size and putting a sheet or towel over it will keep things off the ground. In addition to a long extension cord, have batteries close by so someone can see that something still works. If you are selling a table, don't cover it with stuff. Make sure you line things up in some kind of order instead of just throwing it on a table. I don't even bother going to a garage sale that looks like they just dumped stuff out of a box. Lay stuff out the night before on tables in your garage. Then just have someone help you pull the tables, boxes, etc. out in the morning. The less you have to set up in the beginning the better. Most of the serious shoppers come right away.
As someone suggested, post on craigslist. Be sure to list items that will bring people: baby outfits on hangers, child's bicycle, dorm fridge, crock pot hardly used, ...
Have bags and boxes for people to put their things in. If you see someone grabbing lots of items, offer them a spot behind you where they can make a pile.
Stuff that doesn't usually sell should be priced so cheap that someone will buy it because they just can't resist that ridiculously low price. Who really needs another coaster, for example.