Lots of signs, advertise in the paper if you can also
Make sure your signs are visible for people driving past, and are clearly written. Adding balloons can also help.

Have a float with a few different notes in there and lots of small change. Make sure you take note of how much is in there before the sale begins though.

Price as much of everything as you can as it makes it easier for people. And do take in mind that people expect everything for nothing, so try and stand your ground as much as you can or compromise.

Try and have some plastic bags on hand and some newspaper to wrap delicate/fragile items.

Be wary of early birds (buggers!)

Ummmmm I cant think of anything else atm. We have had several successful garage sales though