Most important thing is transparency. Don't be like a certain retailer and say "oh items are coming next month, no wait next month, no wait the one after that", it's all just hot air. If you have delays be honest about WHY there are delays. Doing something to prove that the delays are real (like showing invoices proving you actually ordered products) goes a long way to helping ease customers minds.

Many people wont put up with excessive lateness or rudeness from a merchant. Learn the value of the motto "the customer is always right". Don't treat customers like they are idiots or only have value because they can give you money. The best thing a customer can give you isnt their wallet, it's their loyalty and praise.

drifand has said a lot of things worth paying attention to as well, you might not have experienced the worst of things that an online retailer can do but we have, and because of it even though there are now 4 or 5 other AU online stores we could buy from we still dont have much trust in them.

Oh yeah, and while it might be tricky to do a lot of us would like deposit only preorders, we might hesitate to give all our money over for an item that wont be coming out for 6+ months but we'd be more willing to pay say $10 to secure that item. This does require you having the capital to pay for your stock in advance and not getting reimbursed until the customers item is ready to be mailed to them, but it's much better then a certain retailer that takes customers money and only orders afterwards at the last minute.