I third the StripStick. If you cannot afford it, look for unfinished half-round molding in a hardware store (not all places have it; you do *not* want three-quarter's round). You don't even have to cover it with fabric (the StripStick is covered with muslin). Works great for keeping the seams straight, and you can press either to the side or open.
Here's a link to the type of wood you would need:
http://www.homedepot.com/buy/america...l#.UQ7Y2h3hq0I