I've used one of these for years on my 75 Yamaha.
http://www.cabelas.com/product/boati..._SEQ_104655780
It does a good job and is easy to use as long as you are going forward. Reverse is a pain in the butt.
My new 140 Suzuki 4 stroke will have a trolling mode switch on the dash where the engine rpms can be raised or lowered 50rpm per click. I rarely troll over 2mph.