Go to Joann's and buy some interfacing for knits - I found a link here, to give you an idea of what should work. I bought a yard and used my coupon, so the cost was a little over $2. Cut your interfacing into strips 1.5" wide like this: Now- simply place the strip of interfacing over two pieces of batting that are butted up against each other. Iron it in place. Keep ironing strips to join the pieces together - overlapping the strips a bit when you are starting with a new piece. The instructions which accompany the interfacing suggest the use of a "pressing cloth" to adhere the interfacing - I found that this was completely unnecessary (thank goodness - throwing a wet rag on batting doesn't sound like a good idea anyway).
For my Improv quilt, I put strips of the knit interfacing on the back of the join as well - just to be extra thorough and because I had so many pieces leftover. It's probably sturdy enough with one layer though.
So, I have to say that I didn't really "make up" this idea - I had seen ads for "batting tape" in the back of quilting magazines that cost about $15 a roll. If I am going to spend $15 just to fiddle with a bunch of batting scraps, I might as well just buy new batting - queen sized runs about $23, you know? But for a little over $2 - it's much more economical (and easy!) to piece batting scraps together and it's environmentally friendly, right? Plus you might not be so plagued by batting pieces all over the house - feeling consigned to make bibs and mini quilts just to use them up!