I have always done all of mine the secret is to have them as Kellie says wrapped securly with their feet hanging out of course, you can buy special powder from places like Jolleys and most pet shops called blood stop its a yellow powder and if you do happen to nip the vein a cotton bud dipped in the powder and held against the cut nail, it stops almost immiediatly, the trick is with light coloured nails you can clearly see the vein so never go too close to it, dark nails just nibble until you get the right length,
some of mine boys especailly never mind the nail clip and are quite relaxed about it all so I dont need to wrap them up but the girls seem to think oh no here she comes the mad nail snipper,
our vets charge about £7 per pig which with my little lot is out of the question, so be relaxed and be ready its such an easy job once you know how,