Choose an effective title and subtitle
Titles such as "for sale" or "assorted furniture" are not that great. A title such as "Table and four chairs for sale" is much better. Including "for sale" at the end is good because it makes it more obvious to people scanning the View New Posts list.
Include photographs
If you don't post pictures you will nearly always get at least three people asking you for pictures. Don't offer to send them by email, just include them in your post. Instructions are here: How to post images to the forum
Describe your items in intimate detail
If anyone wants to buy it, they'll want to know all about it, and if you post it once you don't have to email it to everyone who asks about it. This is especially important for hi-tech equipment like computers.
Why you're selling it
Include this information. If you don't, then suspicious people like me will assume there's something wrong with it that you're not telling us.
Computers
Please go to an online computer shop, configure a computer that has all the stuff yours has (including the power supply and keyboard and mouse and monitor!), and then list all those prices added up so that the users have a fair basis for comparison. Otherwise our very tech-savvy crowd will eagerly tell you how your computer is overpriced and why. Don't bait them, they're hungry and bored at work. Don't forget, if you have installation cds for your software, that's an important plus.
Close your advert
Close your advert when your things are sold, the job is taken, whatever. This prevents people from asking about it later, calling you, and so on. To close an advert simply click the "Close ad" button to the top right of your advert posting. Adverts are automatically removed from the system approximately 24 hours after being closed.
Summary
Following these steps will save you trouble and repeated checking back. People should then only post to the topic or PM or call you if they're really interested in the item. That makes our classifieds more efficient and more useful for buyers and sellers. See also the tips for a good advert on eBay - the same principles apply to TT and everywhere else on the Internet.