Let's start with perhaps the best tip anyone will ever give you - when you ask someone for help, always imagine that the other person's time is just as valuable than your own. Just because somebody doesn't charge for their time doesn't mean that it isn't valuable - there are always other things they could be doing!
What does this mean in practice? It means that you should provide all the relevant information up front - if you don't, then at best the person helping you is going to come back with questions, and at worst they could waste their valuable time looking up things that you already know. Can you imagine how demoralising it would be for them to discover that they'd wasted their time?
When you provide information always make it clear where that information came from: if it came from a certificate and is confirmed by census records then it's probably true; if it is based on a family story that has passed down the generations then it probably isn't.
Just as important as the information you've found is the information you haven't found. Where did you look? What exactly did you look for, and over what period? There's no point saying "I've looked everywhere" because everyone will know you're lying - you need to be specific, otherwise someone else will have to repeat your searches, which isn't a very good use of their time.
If you're a beginner don't be afraid to say so. You're far more likely to get a helpful response if you're honest, and often the person helping you will show you how to do something so that next time you'll be able to do it yourself. Now wouldn't that be great!
Of course, sometimes you'll ask a question which is so common that it has already been answered elsewhere (hopefully if it has been answered on the forum you'll have spotted this before you request). In this case you might be referred to an article in the LostCousins newsletter, or to another website - which either gives the answer, or explains how you can find it yourself. Either way, it's a great opportunity to improve your own research skills by following up the suggestion.
Choose the title of the post carefully. There are two keys reasons to think very carefully about the title you choose - one is that you're more likely to attract the attention of the members who are best able to help if you can be more specific about the help that you need; the other is to make it easier for members who have a similar problem in the future to find your question - and the answers that it prompted. An example of a good title would be "Looking for non-conformist baptism in Essex c1750".
Post it in the most appropriate forum. If you're looking for a baptism in Norfolk don't post your query in the England forum - use the Norfolk forum instead, because that's where you're most likely to find the members who are experts on Norfolk records and where to find them.