I like the type that velcro on so they are easy to clean. Here's an example, but there are others that work just as well:
I use zip ties to help secure them to the outer fork tube. Its importatnt to remove them and clean them after every muddy ride. Our mud is so gritty that almost every mud ride would ruin a set of fork seals. After starting to use the seal savers, I haven't had to replace fork seals in several years. Since I clean them after every mud ride, they haven't scratched the fork tubes. I've heard of others that have experienced scratched fork tubes from not ever cleaning them.