Choose a nice backing fabric for your quilt.
Although we don't often see the back of the quilt it is always a nice surprise to turn it over and find something lovely to look at. Plain fabrics look fabulous and will highlight the quilting but I don't recommend Sheeting. Although sheeting is an economic choice it can also be troublesome to longarm quilt. Quilting fabric will give best results.
Fleece is also an option that feels wonderful as a backing. I can quilt fleece backs but I do request that a light batting is also used to help stabilise the quilt sandwich because some fleece materials are very stretchy.
You can purchase 108" wide quilt backs or you can buy standard width fabrics and piece together to achieve the desired width/length for your quilt. In either case a bit of preparation is required. Wide backs usually always need to be squared and ideally are best pre-washed before loading on the longarm frame. They can also be stiff with surface finishing and quite wrinkled off the bolt. Standard width fabric will need piecing together, ideally with a narrow width zigzag, deams pressed open and the selvedges removed.
Check out the Quilt Preparation page in the link below if you wish to do this yourself. Alternatively I also offer quilt backing preparation as an additional service.