MichaelS, I read on this site that you should gradually get the dog used to the trimmers and the process. This worked for me. The first few times I did not attempt to cut the nails but just let the dog sniff the trimmer and gave her treats when she did. I put the trimmer on the ground with a few treats on it, and let her eat them off the trimmer. I sat with her, relaxed, and gently touched the trimmer to her feet and nose, and gave her treats when she relaxed and sniffed it. Basically, we desensitized her to it.
When it came time for trimming (about the third session of this) I clipped one claw quickly and not deep at all, to avoid any chance of quicking her, but to get her used to the motion and the "click" sound. She got a treat and praise when that happened. I put that treat on the ground between her legs so she had to focus on getting the treat and not what I was doing. In short order I was able to progress to the other claws. We remained relaxed throughout the process. The key, as with most training, is showing dog clearly what is expected, in a calm, non-threatening, but commanding way.
If you dog is older and has developed a "thing" about this that may make it harder. But I think that defusing his fear of the trimmer is the first step, and making it fun for him removes his anxiety.
Liesl, b. 1/1/11