Spend a little bit of time looking through ads in your local Pennysaver to see if anyone has already listed what you're looking for (golf clubs, old bicycles, TV, etc).
Drive around the neighborhoods to scout areas where the sales might be. Remember, being early means greater chances of snagging that item you're looking for.
Its a Garage Sale, so do not be afraid to haggle the price. Sellers usually set a price but are willing to negotiate most of the time.
Always bring cash as opposed to checks, most Garage Sales could and would only accept cash, so be sure to bring enough for your budget. Smaller bills and pocket change are helpful.
Be prepared for the occassion. Wear comfortable clothes and shoes and do not be afraid or embarrassed to rummage through things. Trinkets, books, clothes and other smaller items often get strewn together, so you'll never know what's there if you don't sift through.
See the full list of garage sales in your neighbourhood.