I'm not the only one dealing with an unseasonably cold Spring. Annette, in Virginia, wrote to me asking for frost-protection suggestions for her garden. I've prettied up the links, and am posting my reply to her to help anyone else looking for ideas:
I use Wall-o-waters on my tender plants. Home Depot had some this year, and sometimes Wal-Mart does too in early spring, or they can be ordered online. Okra was the only thing that froze one year (29 degrees June 7th) inside the W-0-W's; tomatoes, peppers, and eggplants survived. I wrote about them here.
I haven't used any kind of tunnels (she asked about them specifically), because with my S-shaped beds it would be difficult to make them. I am, however, thinking about redoing the garden into long straight beds just so I can make tunnels to stretch my season, after reading this Eliot Coleman article in Mother Earth News.
I don't think sheets (that's what she's using) provide enough protection when I have to cover things overnight. I watch for single-layer (not quilted - takes too long to dry if wet) bedspreads and draperies in thrift stores or garage sales. I once found a hideous red velvet bedspread that worked great for years. I like draperies because the pleated top section is heavy enough to stay put on the windward side and then the fabric spreads out nicely, with a heavier hem on the other end. I remove any vinyl liners and just use the fabric part (the liners end up covering the compost). I drape them over plant cages, and wrap them around tall staked tomato plants, sometimes using clothespins to clip ends together or to attach to the cages inside, and bricks to hold down the ends. Photo here.
I have some wire fencing bent into bracket-shapes [ , wide enough to cover my wide beds about 5" high. I put a sheet over one of those to shade carrot seeds for better germination (blue sheet here). They can also support the heavier coverings if necessary in early spring when plants are still quite small. They work great if I have to cover cucumbers and squash that have just started to run.
That ought to give you some ideas to work with. Good luck!