I generally salt down any leftover bait whether it be fish, mussels, shrimp or worms. As with most things, I keep it very simple and what I do is probably not as good as what a top notch recipe would do. I simply put the bait in a bowl, make sure it is thoroughly covered with salt (regular or rock salt) and then I put it in a Ziplock bag, again making sure the bait is thoroughly covered with salt. It then goes into the freezer.
Lately I've begun taking any mackerel I've kept for bait and sugar-cured it. I fillet the mackerel, lay it skin-side down and sprinkle sugar on to the meat portion making sure it is thoroughly covered. I put salt into a Ziplock bag and layer the mackerel fillets one on top of each other with salt being added to each fillet before the next fillet is put on top. I usually use about 2-1, salt to sugar portions. Each Ziplock bag goes into the freezer.
As said there's probbably a better way of doing it but I've been very pleased with the results and given the price of bait today (especially pile worms and blood worms) it's good to be able to use what was left over.