Most of my friends and family know that I adore yard sales - both having them and going to them. We hold our own annual yard sale every spring usually with several like-minded neighbors. We had our sale in May and I made about $700 which definitely made it worth the preparation!
I like having a garage sale both to push myself to clean-out stuff we aren't using (or plan to use in the future), and to make a little dough in the process. And, I think I may be a store owner at heart so this is a way of getting that urge out of my system once a year.
To prepare, I have a yard sale "sell pile" in my basement at all times. It generally starts growing a week after the last yard sale and consists of about 10 large opaque plastic boxes (supplemented with cardboard at the very end) stacked in our storage room or under the basement stairs. I sell unused kitchen gadgets, toys, books, furniture, clothes, comforters, decorative items, etc. Pretty much anything someone might pay at least a dollar for!
I learned most everything that I know about sales from my mom who is the queen of yard sales. She has had one nearly every year for as long as I can remember. I have a feeling I am following in her footsteps (much to Jim's chagrin).
Here are my general tips on holding a yard sale:
1) Put prices on everything. It's a lot of work but many people will not buy if they have to ask how much an item is.
2) Dollar & up. Price everything at $1 and up. Group small junk items (like toys from McD's) in a clear plastic zip-loc bag and price it at a dollar or more if there is a lot in it. Generally I make my items pretty cheap because I would rather sell for a little less and sell a lot.
3) Neighbors. Your neighbors will prove their infinite value once again if they join you in having their own sale. People are more likely to hit "multi-family" and "neighborhood" sales.
4) Advertising. An ad in your local paper will generally pay-off because it will bring in the most die-hard yard salers. It is currently $30 for an ad in the Washington Post. A free posting on Craigslist is also a good idea. Take advantage of it's free-ness by including pictures and a laundry list of items available. Easy to read signs on the major roads near your house are absolutely necessary (my own sample above which we reuse every year).
5) Start early. Your sale should start no later than 8 a.m. You'll make about 75% of your money before 10 a.m. so make the most of the time before then. Even if you have an 8 a.m. start time, be ready by 7 a.m. because you'll inevitably have early birds.
6) Change. Have a lot of 1s and 5s on hand (at least $100 total).
7) Avoid competing with large group sales. In Arlington, there is a large community sale on the first Saturday of every month. I generally avoid the first weekend of the month although it was unavoidable this year, and I think we definitely lost out a little as a result.
8) Stick with your asking prices on large items until 9 a.m. If you don't get your asking price before 9, then be willing to bargain.
9) Clothing. In my experience, clothes and shoes sell - especially men's clothing. Put prices on major items like coats, suits, and nice dresses. For the rest, make a couple of large signs with the prices like $3 each piece or 2 for $5.
10) Store style. Have as many tables as possible even if it means a piece of plywood on some saw horses. Keep like items together so all kitchen stuff on one table, electronics on another, etc. Put the best stuff and furniture near the street so people can see that you have good stuff and will be more likely to stop.
We have a general rule (well, Jim has a rule) that nothing comes back in the house. He loads up our SUV with whatever is left over at the end and brings it to Goodwill or another local charity. He comes home with a tax receipt (an added bonus), and usually a big bag of McDonald's!
At lunch time (also known as quitting time), it feels great to have a big pile of cash replacing the big pile of stuff I had at the beginning of the day. We like to find something to use the proceeds towards such as the new patio furniture we bought with last year's proceeds. This year we didn't have an intended purpose but we have already spent that money many times over - mostly on Thai food delivery and babysitters!
and JCPenney came through for me (once again) for about $300! 
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(a Christmas gift from my parents)! I think it adds a lot of charm to the room and makes visitors feel like laying on the sofa and reading a book (or a decorating magazine!). The curtains look like linen and have really interesting vertical stitching on each panel. They look expensive but they are machine washable which is critical for an outdoor application. I am still deciding whether or not to use the tiebacks which came with the curtains (added bonus!).