I have a 2004 150 HPDI and prior to a planned fall day out fishing I change the gear oil, fill my oil reservoir, change my water separator, add Stabil to my fuel tank and fill my fuel tank. This way I know I have run stabilized clean fuel through. I do my kicker gear oil at the same time and change out it's four stroke oil. I aways flush my engines with fresh water and keep the engines legs down.
Since I use my boat occasionally in the winter I do not fog my engines. If I don't get out in 6-8 weeks I will run the engines, at idle, on earmuffs. If I wasn't running the engines in the winter I would fog them. The HPDI has a large black air cover that can be easily removed, and with the engine idling, fog this air intake, pull the plugs and spray some in that, replace the plugs but don't connect the wires and turn the engine over a couple of times. Attach the wires to the plugs. I also spray a light amount of Liquid-Film over my engines every 6 months or so.