Its been a long time since I smoked salmon (probably 25 years! My wife HATES any seafood, so I don't cook much of it), but here's what I used to do...
I would brine the fish in a brine (doh!) that included pineapple juice. I can't remember how long, but at least a couple of hours. Then I would put the fillets on the smoker rack and top them with a layer of sliced onions. On top of the onions I would put a generous layer of dark brown sugar.
At the time, I had an ECB, and I had no idea of what temperature I was smoking at, but the salmon came out very good. No one ever complained!