You will want to buy a large tarp from the sporting goods section at Wal Mart, and bungee or tie-down straps from automotive.

Lay the tarp across the roof of the vehicle, load luggage in a fashion where is it not prone to shift or slide.

wrap the tarp around the luggage, securing ends by tucking them under the luggage, then use the bungees to strap them downyou will want to run them where they overlap and run verticle and horizontal
ex. strap 1: run from front to rear strap 2 run from left to right overlapping strap 1, strap 3 run front to rear etc… I would recommend using at least 4 straps.

Or you can rent one of those little U-haul buggy like trailers to pull behind.