I use pins to mark the middle of each layer, taping thethe middle of my backing tothe middle of my 6 foot banquet/craft table. Then I align all the layers, doing my best to get the centers to match in both directions.. Once it's all lined up, I run a line of sewing pins along the edge of the table holding them in place. Then flip back the top and batting, spray baste, smooth the batting down, spray baste, and then smooth on the quilt top. Then I move the entire quilt down, tack it with tape again and repeat until I've finished one half. Then I reallign the pinned edge to the table, and work on the other half from the other side of the table. If it's a big quilt, I usually enlist my husband to help hold up the extra layers while I spray and smooth them.
Once I'm done spray basting, I like to either stitch in the ditch or use a basting stitch to make quadrants on the quilt, so the layers are less likely to shift or wrinkle. A quick trip around the very edge of the quilt is also helpful, It holds the edges together and makes quilting the borders so much easier.