Some other miscellaneous advice would be: Resolve early on that anyone who presents you with a $50 bill, for anything less than about a $42 purchase, will be regretfully but firmly turned down. Once you use all your change to make change for that one person who hands you a fifty, your entire change-making balance will be off the rest of the morning, and you’ll lose sales because you can’t make change.
So don’t take fifties.
Or checks.
I shouldn’t have to tell you that. 
And anyone who gives you a twenty for a fifty-cent t-shirt may be freely cursed, with voodoo dolls if you like. Because classy Garage Sailors do not do this. We know to bring quarters.
Use those colorful round sticky dot stickers for price tags instead of chunks of masking tape. They stick better to things, they’re easier for buyers to see, and they’re way easier to work with. You can also color code them: red for “will not negotiate on price”, orange for “will come down half on price”, yellow for “get rid of at any cost”, blue for “give away free if necessary”, green for “give to [person] who wants it if it doesn’t sell”, white for “throw away if not sold”.
Put an ad in the “Garage Sale” section of your newspaper. Put a couple of inflated balloons out in front of your house with a “GARAGE SALE!” sign to make it easier for the folks who saw your ad and who are looking for your house to spot your house on the block.
Don’t bother putting crooked hand-lettered tiny 9x12 posterboard “garage sale” signs on telephone poles and whatnot around your neighborhood, hoping to attract casual drivebys, because casual drivebys are going too fast to see your sign, and aren’t in the mood for/don’t have time for your garage sale, even if the wind hadn’t flapped the sign around so you can’t see the address. Put your faith in the ad in the paper; the folks you want to attract, the serious Garage Sailors, get up early on Friday and Saturday mornings and peruse the ads, making little maps and game plans. Those are the people who will come and tote your junque away for you, not the casual drivebys.
Plan on having at least a Friday and Saturday ad. If you live in a big garage sale town like Decatur, Illinois, a Thursday extension will be appreciated by the cognoscenti. Also plan on a $1/bag sale after noon on Saturday.
Definitely have a Rain contingency plan. Rescheduling is generally Not Done. All those serious Garage Sailors didn’t spend an hour making maps and timetables just to get to your house and find you Closed For Business. It makes them very cross, and they won’t come back next week. So you need a place to put your stuff if it rains. Garages are good (duh). 
Cover anything that’s not for sale (lawn mower, bicycles, petunia planters, etc.) with bedsheets. If you move your venue because of rain, like to a neighbor’s garage, make SURE it’s clearly marked, with a humongous sign and lots of balloons, "GARAGE SALE HERE ----> "