Good ideas Petfriendly & Prin.
It's important to teach him that being calm will get him released and being pissy doesn't work.
Every dog is different and this may not be related to any past experiences - most animals aren't thrilled about getting their nails done at first.
Also, dogs do remember lots of things for a long time, I think the trainer meant that in order to make an association for good or bad - the person has a very short window of time.
Some dogs are better sitting and just offering you a paw for trimming. Other dogs do better lying on their backs for trimming. We teach the 'love & trust role' for nail trimming too. It places the dog on their back - submissive position - and lets you practice working their feet and nails. When you start to actually trim the nails just take off the tiniest tip and praise. If you do his nails every day then he should start to get over it. If he complains or gets mouthy then a correction is in order but go right back to being calm. If he is on his back and gets mouthy you can actually take his own front paws and when he heads for you - you hold his own paws infront of his mouth so he can get nippy on his own paws not you. He'll get board with that game fast.
Love Them & Lead Them,
~Elizabeth & Doug
Dog Training the Way Nature Intended