I've taken out-of-towners on this tour twice recently and went once on a school field trip forever ago (yep, our school thought it was a good idea to take a bunch of local city kids on one of these...it wasn't).
Get a reservation. Show up early if you want to be able to get a window seat.
Be prepared to quack and to be called out if you don't quack. I always thought it was super dorky when these guys passed me downtown, but it's less so when you're doing the quacking.
Don't be afraid to volunteer when they ask who wants to drive the boat. I sent my nearly-thirty-year-old friend up to drive, and it looked like she had more fun than the seven-year-old who went before her.
I'm docking a star because as a native, I didn't learn anything on the tour. The information was pretty general--in fact, for 90% of the tour, I pointed out something to my friend, then the driver said the same thing, then I added more information. But I have been told I'm like a walking Google Maps for Pittsburgh, so maybe I should look into a second career with Just Ducky...
Final note: Take a trip up to Duquesne Incline while you're at Station Square to see the city from Mt. Washington. Only $3.25 (exact fare only) to see one of the "The 10 Most Beautiful Places in America" (usaweekend.com).