Clearly Describe the Problem
This is easily the most important tip that I can give. Too often I've seen posts that don't clearly describe the problem, and it takes a while to nail it down. Before you post your problem, take a moment to collect your thoughts. What have you done to try solving the problem? What have you found while researching the problem? Where does the error occur?
It is extremely important to be absolutely clear about the problem you're facing. Let us know precisely what errors you're receiving, and what lines. Don't be afraid to copy and paste your error messages. Which problem description is better?
"Cannot send headers in foo.php at line 15. Headers already sent by foo.php at line 7".
Obviously the second problem description is going to get more responses.
Don't Post Like an IM
Which of the following would you rather read?
Posting on the discussion board is completely different from IMing your buddies. You're here with a specific problem, and there's no point in making it harder to understand the problem with your leet speak. Use the best language and sentence structure you can.
Use [code] [/code] tags
This is at the top of every forum posting, yet we still see people not using the tags. Use them! It makes your code much more readable, and will be more likely to elicit a response. In case you haven't noticed the pattern here, it's extremely important to make your posts readable by someone helping you. These people are taking their time to give you free guidance and assistance, so you should make their job as easy as possible!
Post as Many Details as Possible
Post everything you can to allow us to help you. Use details when describing everything. Post your code! Include your database schemas! Everything you can give us that's related to your problem is great. The more information we have, the more likely we'll be able to come to a solution.
DIC has a system you can use to thank a poster for a response. Use those little green thumbs up "This post was helpful" links. People like to know that their time is appreciated, and that's one of the best ways you can thank them. The more a person feels they're appreciated, the more likely they'll stick around and continue to answer questions.
The last piece of advice I have is actually an important one. You need to be patient when posting your problems. Posting a problem, and then coming back several hours later and posting "Anyone!?" is not going to get you help. If anything, it will push people away from wanting to help you. It's important that you give everyone time to read your answer and give you a correct response. It's better to get one good response than 20 rushed ones.
There are many other ways to get people to help you with your post, but those should be enough to get you started. I'm sure there are other ideas out there as well, so if you have something I haven't posted, put it up!