1. Starch the back first, that helps.
2. Smooth it out using my hands, a metre-long ruler.
3. I always baste with someone to help me, so we both make sure we've got it pulled taut over the floor and then tape it down with masking tape. If you have a low-loft carpet, you can pin it instead. If you have a high-loft carpet, you probably don't want to be basting over it.
4. Repeat the smoothing process when putting the batting on, and again and most importantly when putting the top on.
The biggest quilt I've made is 89", and I've never had any problems with pleats or wrinkles in the backing. Admittedly, that particular quilt took rather an involved basting process as it was too big for the floor space, but that's a different story, and you get the idea. I'm probably an over-careful baster, but I'm happy enough that way and I would hate to spoil a quilt through a mistake at that stage.