First, it begins with you. If you have a legal question, no matter where you ask, you must include the state that you are asking about. This will prompt more experts to answer your question. So if you have a child support, ticket, criminal, or civil law question, start your question with the state that you live in. Never leave that out or many experts will just look over it because they know you are wasting your breath. State laws vary incredibly.
The second step is to know how to avoid specifics. Never give your name or case numbers freely over the internet. Also know to avoid very open forums such as yahoo answers, as you don't know if it's an 8 year old kid just out to give you a hard time.
Many legal forums such as http://www.expertlaw.com/forums are an excellent place to start asking. Many people in these law specific forums are students of law, ex law enforcement, and former judges. If somebody gives you bad advice it is usually dealt with by others calling it out.
Find attorneys in your area and email them your question (along with asking what their charges are to represent you). Many times, they will be happy to answer 1 to 2 questions, so long as they think you may be willing to give them some business. (If they do help you and seem reasonable, do give them business if you need them!)
Being anonymous can also help asking about troubling legal questions. You can use alias email accounts & ask questions on open forums and to lawyers this way. I would advise people remain courteous though, since they are doing you a favor!