It may not be the most economical but it gets done right. I take my rig to a dealer and have them treat the gas, change oil, lub lower unit, blow out all lines (bilge, live well, etc) and fog the motor, etc. It costs a bit but for me, worth the hassle/insurance it's done right. There's also the bonus of gaining a good relationship with the dealer in case something goes wrong. e.g. he checked my onboard charger for some issues this year and didn't charge for that. I'm no mechanic so for me, there's a value in someone who knows their stuff to keep me running.
An added bonus would be that they know the boat if / when you're ready to trade up and they know it was well taken care of.
sorry for the rambling but.....