If you've got a lot of stuff to sell, practice laying it all out at home. Measure out your space, mark it on the floor and set up how you plan display stuff. This may sound stupid or a waste of time, but presentation is key to bring people in to look at your wares! If you've got a nice thought out display, people will see that you care and treat your items respectfully, and this can help with sales. If you just dump over your bag/box/bin of stuff on the ground with no mind to set up, you may not have enough eyes to even look your way to see what you've even got to sell. Because, really, would you rather buy used stuff from somebody that keeps stuff clean and orderly, or somebody who picked up a random grab of stuff off the floor in their room?
post-it notes or a notepad with easy to detach paper to hand off to people in case they make an offer or want to come back later to redeem a bid or special offer/price you told them for items you were going to sell is a good idea as well.
Most everybody has a smartphone nowadays. Most responsible adults have credit cards and/or PayPal accounts. Consider taking PayPal for payments, and/or requesting a card reader from PayPal so that you can accept credit cards. The fee they take out is nearly negligible when selling large ticket items such as figures.
Large inventory of stuff, lots of things to keep track of? Print up a list of what you've got and cross off as the night goes on.
Sitting on the floor sucks. Bring a chair of some sort or something you can turn into a makeshift chair, something that's easy to sit and stand up from. You'll be getting up from it a lot.
Lay out everything in a way that you can see everything, yet if you need to get up and out of your spot, you're not hopping over any of your stuff. Worse case scenario, somebody tries to snatch something from you, HOPEFULLY you won't need to give chase but if you've got somebody with you, you can either send them to get help or go yourself. Never really an issue in all the years I've been selling, but you should always expect and plan for the worst.
Collectors/Resellers: First of all, I sympathize with you, but at the swap meet you're going to see a lot of unhappy/unfriendly faces. If you've got a lot of mint NIB figures/games/comics/whatever, and are organized well, realize there's going to be haters that will accuse you of being a dealer. My only suggestion is to price fairly, check your prices against eBay/Amazon/online retailers and price accordingly.
Also, you're not the only person who does this, so don't be offended when you come across a "smart" shopper that tries to make you look bad by saying x is overpriced, and in a very loud, extravagant way as if announcing to the whole room. Unless you've blindly thrown a price on something way above market (again, check those online stores for prices), they may just be doing what you've already done - buy low, sell high.
If I can think of more I'll post again