It's a simple project that involves sewing lengths of PVC pipe into the two ends of the bed sheet for strength and stability. As a bonus, you can easily roll up your projector sheet for storage.
Some eyelet grommets in the top end of the sheet and eye hooks screwed into the ceiling ensure that you're ready to hang and watch—no push pins required.
You could even take a cue from Instructables user TimBTodd and have eye hooks ready in a few rooms. Good for extra-broke people with projectors in every room—or those whose projectors are portable.