Our multiple's club does two garage sales a year. We have a general suggested pricing spreadsheet that we use as a guideline. If you'd like a copy, send me an e-mail address and I'll send it to you. (I tried to copy and paste and it didn't work here)
For change, I usually start out with maybe $30 in tens, $30 in fives and $50 in singles. I round off all my prices and totals so there's no coin involved. I keep my money in a fanny pack instead of a cash box so it's with me constantly and not sitting around. I maybe keep $30 in it, with the remainder in the house, by the back door.
Keep the door to the house locked as well. One person may try to distract you so a pal can run through your house without you knowing it.
Keep the cordless phone out in the garage with you as well so you're not running in the house to answer it with a garage full of shoppers.
Another idea - if there's things in the garage that are not for sale, cover them with a sheet, drop cloth, tarp, etc. I don't need someone to scope out my DH's tool cabinet then break in while we're on vacation and steal it.
If you can team up with a neighbor, do so. Combine items to be sold at one house and have the two or three of you available to rotate and man the sale in pairs.
And, finally - no one gets to go inside your home to use the bathroom. You can't be in both places at once and your primary goal is to man the garage sale.
Best of luck and have fun!