Coverage is important. When you work with a Realtor who posts your property to the MLS, it is automatically posted on several of the most popular online sites as well including Realtor.com, Zillow, Trulia, Redfin, and every other site that allows buyers to search the local MLS.
To get as much coverage post to several of the sites below. Your area might have specialized sites as well.
Zillow. Free. Click "Submit Listings" on the right side of the home page and it will bring you here:
Trulia. Free. Click "Home." Towards the bottom of the drop down menu you'll see "For Sale By Owner" which takes you here:
Craigslist. Free. Consistency is the key with craigslist. Posting every day or every other day keeps you at the top of the list and in front of buyers. Don't be surprised if your craigslist ad attracts a few wholesale buyers hoping for a good deal.
ForSaleByOwner. Cost. They offer an MLS option.
FSBO.com. Cost. They offer an MLS option.
Owners.com. Free for Basic. Cost for other packages. MLS option offered.