Have a well thought out plan! That is my biggest tip. Don't jump in head first without considering as many aspects as you can and planning how you will handle them. Think through your marketing plan, your product line, your sales strategy, your design, and your financials.
You can get yourself into some serious headaches if you do not think through the abovementioned elements. Understanding yoru financials is so important but is often overlooked. This involves pricing (and reviewing competitor's prices), quantities, discounts, and even just sales volume you will need to cover your expenses. You have to understand what numbers you will need to hit in order to breakeven or make a profit.
After that it is all about how you are going to reach those number, how you will drive sales, market to your customers, and what your site design and brand will look like.
Another very important consideration - find an ecommerce provider whose product/service makes the above things much easier with built in functionality and service offerings.