Hi. I do sympethise with you on this. Ds (25 months) has protested about having his nappy changed for, on and off, about a year!
When he was a bit younger, distractions - particularly with interesting things of ours he wasn't normally allowed - worked pretty well.
Now, as another poster suggested, talking about something - engaging his interest in a conversation - sometimes works.
Two other things we've had more success with lately (and only really since ds turned two) are, firstly, clearly saying to ds: "You've done a poo and need a nappy change. I'm going to the changing mat. Let me know when you're ready." I potter down to his room and sit next to the mat, nonchalantly getting a nappy sack and wipes ready and pretending to read a book, and quite a lot, he quickly yells, "I'm ready!" and hops on the mat. Dp and I were amazed when this started working.
The second tactic we use when the when-you're-ready one fails (these tricks only seem to work for a bit, don't they?) is, "Right, you've done a poo. Your bum will get sore if we don't change your nappy. You can come to the mat now, or I'll count to three and then I'll put you on the mat." (All said calmly but like you mean business.) This is the most effective tactic at the moment. As soon as we say, "One ..." ds usually runs to the mat and enthuses that he's ready. Apart from the other day when he said, "... two, three!" back to me.
I do understand it's a pain in the arse when they won't cooperate for a nappy change. I have changed many a nappy through gritted teeth and while taking deep breaths - you don't always have the energy/creativity to divert them. Ds is showing quite a lot of toilet training readiness, and I do say to him that if he really doesn't like nappy changes, he has the choice of starting to use the toilet. Am waiting for him to say he's ready on that score, too!
Good luck.