I use mussels quite a lot.
If you collect them yourself, then I suggest you shell them the day before and keep them all in a container in their own juices.
When you need to use them either thread them onto a hook, or simply just tie them on to the hook, but whichever method you use, you will have to use bait elastic, or they will simply fly off during even a gentle cast.
I usually always use them along with rag or squid etc, but have also caught fish with mussel as the only bait.
They freeze very well, and again just shell them and shove them in an airtight container in their juices, then just chuck them in your freezer.
There is also good thread somewhere on this forum on freezing them in sausage shapes so they can be placed straight onto the hook part frozen. Another guy said he hooked them all onto quick release hook snoods and then froze the whole lot like that. Then he simply unclipped the old snood, and clipped on the ready baited one.
I usually catch Cod, Coalfish and Dabs with mussels.