How to stop a necklace slipping?

Crimp beads? And then crimp bead covers to make it look like a regular bead. Pic here:

Tutorial here: http://www.wikihow.com/Use-Crimp-Beads

Depends on the type of chain you have though.

As tayto said the only way you can do it is by securing the locket so it is at a set position on the chain. My tiffany necklace is 'set' and I don't have any problem with that swizzling round but my normal everyday work necklace with a loose chain I do. I did think about gluing my 'everyday' necklace so the charm bit wouldn't move but was reluctant as it's platinum and was worried I could ruin it (and it also has sentimental value as it was a present from my mum). I might look at the crimp beads to secure mine too!

I just looked them up as couldnt get my head round them ha. Its a good idea but sadly wouldnt work for my necklace, i think i'll just have to put up with it slipping - it's no biggie really, I just always wondered if there was a way...

A short chain with a weighted bead attached to the clasp should act as a balance and keep it in place.

Oh that's an idea celticcurl, I could try that, ta!!

I'd steer clear of crimp beads for anything other than snake chains really (and even then personally I wouldn't use them on chain).

As cellticcurl says, putting an extender chain on the part the clasp attaches too should help.