My daughter and have held an annual garage sale for the Susan G. Komen Breast Cancer 3-Day for the past 5 years and have made some really good money. Here are a few suggestions that have worked for us, but be prepared to make adjustments for fit your area, laws, etc.
1) Put the info in all local papers and check to see if they have a Community Page that lists all upcoming charity or special events. Listing is usually free as long as it's written up like a press release. One year, we sent out a press release and a reporter showed up the first day and when the article appeared in the paper the next day, we were really busy. Don't forget to send the release to radio stations also.
2) Be sure to list specialty items such as fabrics, furniture, baby items, antiques, etc.
3) If you have clothing, have it organized by size. We've found that this is especially important with childrens' clothing. Try to hang up as much as possible and be prepared to reorganize throughout the day.
4) We've been very successful selling water, soda and candy.
5) We always have a raffle of some sort that is only for people who purchase something at the sale. Usually I make a lap quilt or wallhanging. Doing this encourages you to talk to the customer about the cause and make some extra money, of course. (Check your local laws about raffles.)
6) Have a set "bottom line" set in your mind before the sale starts and make it across the board. For example: Decide that you will only negotiate 15 - 20% lower than the priced item. If the customer balks at this, gently remind them that this is a fundraiser.
7) Have everything priced! It doesn't matter how many signs you put up saying that all books are 50 cents. The customer won't see it and they'll ask.
8) Be sure to have signs up around the sale area stating that this is a fundraiser. We also ALWAYS have a donation jar near the check out area and if someone's total comes to say $10.50 and they hand us $11, we ask if they'd like to donate the change. (It doesn't always work, but you'd be surprised how often it does.)
9) Have enough helpers. People don't like waiting in line to pay for an item that will only cost $1. They really do watch how efficiently you run the sale.
10) Have plenty of bags, boxes, paper, etc. to pack up items that are purchased.
11) Finally, no matter how slowly the sale is going or how bad the weather is, remember to smile and have fun!
Good Luck!!