I've used Q-Snaps for years (when I need a little support and help). They do a great job of keeping your fabric taut, and if it loosens up a little bit, you can just twist the clamps to tighten it back up again. They also do not leave heavy creases in your work, which makes finishing much easier.
 
The pieces snap together (and pull apart) easily, and you can combine different sizes to make just the size and shape you need. Look for other sizes on my site.