I had good success this past winter and spring with Gro-Therm, perforated plastic sheeting that I got from Territorial Seed. The holes in the sheeting prevent an unhealthy build-up of humidity and heat under the cover. I'll be pulling it back out when our summer is over.

Here is a link that might be useful: Gro-Therm from Territorial Seed

I would go with 4 or 6 mil. I've used both. Some was leftover from a roll we bought for a project (from Lowe's or somewhere). I bought more than half a roll from a guy on Craigslist for $10 or $20, I forget now (or maybe I've bought some twice?), anyway, lots cheaper than at the store.

Something like the Gro-Therm may be easier b/c of the venting, but regular stuff works too.

Here is a link that might be useful: Tales of a Transplanted Gardener