Your Pastor is so lucky that people appreciate him!
It's a privilege to be chosen to give the speech - the rest of the congregation are on your side - and willing you to do a great job (maybe they're a bit jealous that nobody asked them!).
Humour is always a good introduction and lightens the tension - once you've got them laughing, everyone, especially you, will relax.
Find some appropriate funny cartoons.
Tell a funny joke-
Little girl, "When I'm big I'm going to give you some money"
Pastor, "Thank you, Ellie - why is that?"
Little girl, 'Because my mom says your the poorest preacher we've ever had!"
If you're not good at humour - before the day, ask all the congregation to write one things that your Pastor means to them (annonymously) and put them in a container. At the start of the proceedings ask some of the older children to come up one at a time to read them out.
The most important thing is that you be yourself - your sincerity will shine through.
I found this video about appreciation - and its very beautiful. Although it's not about a Pastor it really makes you think what a difference it makes when you say, 'Thank You!' to someone. It only lasts a few minutes.