Here is a hoophouse example:
http://westsidegardener.com/howto/hoophouse.html
If you look down at the first picture of the covered
hoophouse, notice the 2x4s on the sides that he
uses to hold down the plastic.
These clips might be handy (stronger than split
polyethelene pipe):
http://www.atozsupply.com/Snap_Clamps_C106.cfm
(That is a chronic problem: wind creating enough stress
on the plastic cover to pop the end clamps off of the
framework. The pvc ones are rumored to hold better
than split pieces of polythelene pipe.)