The best freelance job board I have found is elance. It’s where I found my first clients, and many great ongoing clients. If I ever need to increase my client load it’s one of the first places that I look.
1. Before bidding, research the client. See what other jobs they have posted in the past. Get a feel for which providers they choose. They may only choose lowballers. If that is the case, you might not want to bid. Or, you might want to lowball simply to get your first gig and get feedback.
2. Check what the client has left for feedback. If they have given low feedback, check out the provider they gave it to. If that provider has great feedback in general – you might want to think twice about working with this buyer.
3. Make sure your bid is original. Don’t copy paste the same text into different bids. That means, REALLY read the project description and address it in your bid. The more familiar you seem with their project, the more your bid will stand out from the rest.
4. If they have a website, visit it and try to casually comment on it if it seems applicable.
5. The tone of your bid should be somewhere between friendly and professional. You don’t want to be too casual, but you don’t want to be too uptight.
6. Don’t sound desperate :) Do sound confident.