It may be easier at first for it to be a two-man job- one to hold the pig and hold their foot still and one to clip. I have a small-animal nail trimmer which I find makes things easier... they look like scissors with a hole in the middle of the blades where you can insert the nail. I have also used baby nailclippers successfully, but the small animal ones are easier to use and cut the nail without flattening it out first, which seems to make a cleaner cut. If you have pigs with clear nails, make sure you are clipping above the quick (vein in the nail)- in clear nails it looks pink. Pigs with darker nails are trickier because you can't see the quick, so trim small amounts off the top frequently instead of doing bigger cuts less often. If worst comes to worst and you are still having no luck, check around your area at pet stores and pet supply stores to see if they do nail clinics... I know a chain in my area (Pet Valu in Canada) does clinics for nail clipping on a monthly basis for a variety of animals, including guinea pigs, and it's probably cheaper than a vet visit!