Link it. Links get noticed. Whether you are buying or selling an item, linking entices players to click. Once they click, you've caught them in your (nether)web. To insert a link into a chat channel (which works in whispers, party chat, guild chat, raid chat and Trade), hold down Shift and left-click on the item. Other players can click on that link to bring up the item's tooltip. You can link anything you can see an icon for: items other players have linked, tradeskill items or enhancements from your own or another tradesperson's professions UI, items being worn by others, items displayed as rewards in your quest log, anything in the Auction House ...
When you want a specific player-made item or enchantment -- again, link it. Don't periodically interject a forlorn "Can anyone do Enchant X for me?" into Trade chat. First, ask in Trade for an Enchanter to link their profession. (Preferably, you'll have used wowhead.com or another database to look up what you want beforehand and be able to specify the minimum skill level the Enchanter needs to be in order to perform what you're looking for.) What you'll see in chat is a gold link saying nothing more than the name of the profession; click that, and you'll see every item/skill the tradesperson is capable of producing. You can scroll through that list in order to look up mats and to find links to the items you want.
Know your mats. Before you start asking around for a craftsperson to make something for you, know what materials ("mats") are necessary. Use wowhead.com or a similar database site, or ask someone to link their profession.
Specify whether or not you are providing mats. If you don't have your own mats, know the going market value (check the Auction House).
Know when to not to try to sell something in Trade. If you suspect other players might flame your item or question your sale for any reason (it's grey, it's a novelty item, it's an item you can buy from a vendor), use the Auction House instead of Trade.
Advertise that you're ready to pay up for services. Don't merely ask for an item or service; be clear that you're willing to pay. "WTB port to Exodar, please" or "Need port to Exo; tipping 5g" earns you a better response from Mages who are wary of port-and-run cheapskates.
Be willing to travel. When you want to buy something, it's up to you to get to the store. Have your hearthstone up and be ready and willing to travel.
Don't fume about non-Trade conversation in the Trade channel. Yeah, it's annoying. Yeah, it's going to continue. Conduct your business (with attention-grabbing links, remember?) and people will respond accordingly.
Read more about using (and abusing) the Trade channel: