Don't simply ask how much I charge for [insert your project here]
This is the all important question, but I'm afraid there isn't a 'one size fits all' answer. Costs can vary depending on the complexity of the work involved and nature of the project. The best bet would be to fire over your details, along with a description of your project and the budget you have available.

Do give me an indication of how much you have to spend
Knowing a budget up front helps give a clear indication of how much time can be spent on a project, and subsequently how much work I can fit into the time quota. A budget figure also gives an insight into roughly how large the client is expecting the project to be.

Don't ask how many revisions are included in the cost
This isn't a super-frequent question, but it's frequent enough to be a bother. Asking upfront about revisions gives the impression that the work the designer creates is going to be 'wrong', and then needs to be 'corrected'. I always create my designs with care, consideration, and most importantly, reasoning behind the graphics, and therefore I don't tend to work on a revision basis. If a change needs to be made that's not a problem. If I feel differently I will always offer my point of view, but as long as it fits within the proposed time quota it's no trouble. However if the changes create additional work beyond the initial budget, advice will be given on any additional costs.

Do realize there's usually a deposit required
I work on a 50% deposit basis, where an initial invoice is generated to kickstart a project. Once all work is completed, the invoice for the remaining balance is created then files are supplied by email. Payments can be made by PayPal or check, though I prefer PayPal - it's quick, it's safe, and it's easy!

Don't send an email full of 'legal speak'
There are times when legal contracts and non-disclosure agreements are important, but it's hugely off-putting when an email needs reading three times to understand the point. It's much simpler and easier to correspond on a personal level.

Do have an idea of my style of work and services I offer
Projects often go a thousand times more smoothly when I'm working with someone who has researched my background and subsequently chosen to hire me because they like my style of work. It allows for a more enjoyable project for me, and generates an end product that meets your original requirements.

Don't send a generic or mass email
Following on from the previous point, there's nothing worse than receiving a generic email requesting a quote for a project, that has also been sent to X number of designers and design companies. Do your research into the work of the designer beforehand and choose one that suits your needs.

Do explain how you know me
Having some kind of referral helps your email stand out, whether you're a reader of my blog, an acquaintance of a past client or someone I've met before, it shows that you're genuinely interested in me and my style of work - Not someone on their first time visit from Google looking for a freelancer.