Make sure the sign-up/checkout process isn't perceived as being too difficult or invasive. - Only collect the information you need to process the order and service the customer. - Try to ask for all of the customer's info in one or two steps.
Are the people visiting your site the people you WANT visiting your site? -Maybe you're not targeting properly.
The customers should be aware of all charges before the checkout. -Give them the final price including taxes and shipping before asking for payment.
Does your site load quickly? -People aren't going to wait around for something the don't NEED
There are plenty of other factors that could help increase your conversion rate. You should scroll through this forum and read some of the previous threads. You might get some ideas.