Plenty of change and smaller bills (and periodically take money into the house), keep the house locked/no restroom, have more than one person there, make it look inviting, make it easy for people to see what you have (be it clothes, dishes, records, books), drag stuff out of the garage where it's easier to see, put some of the bigger nicer stuff out in front so people just driving by are attracted to your sale, don't overprice your stuff, plenty of bags/newspaper for packing, be willing to take offers. Furniture is hard to sell because it's big and people have to get it loaded up and take it home. People don't want sets of dishes anymore. Fabric has always done well for me. Holiday items didn't sell as well as I thought they might. Books have always done well for us.
We've had many sales, made decent amounts of money a few times and vowed we'd never do it again! (Sorry). A few years ago we had a sale, had MANY people walk through, had really good prices and people were looking for specific things like collectibles of certain things like matchbox cars or Barbie or Star Wars. The first day, always, we've had dealers swoop in right away and take anything that could be put into a flea market or antique mall. When the weekend came, it was families looking for usable items (so that's good). But when it gets to the point you have good turnout and good prices and only make a couple hundred dollars, it's not worth it for us anymore. It's a lot of work to get it all organized. I've mostly seen nice people at my sales. I don't have any memories of anyone trying to rip me off or take anything. But I do sit where I can see everything/everyone. Kids are cute when they come and have their own money.
Have fun! And good luck to you!