To add to the straps on my carrier, I got some flexible things from Maplin: they're a like jumbo glorified version of those little wires you get with sandwich bags. They're covered with a soft rubber so they don't damage your rims, but hold things down very tightly. They were about £2.00 a pair on special offer.
I found that toe-straps worked better than the straps that came with the carrier anyway.