Here are the routine things I do to try and prevent things from going bad with the lower unit and power head.

Power Head: I rarely run WOT unless its tournament morning or I am late for weigh in. General fishing I try and vary the RPM's and basically take it easy. I over maintain my boat. I do full tune up to the boat about every 5-6 trips or so. It has gotten to the point I can, load it with grease, flush it with Merc. power Tuner change plugs and be done in about 15 minutes. I have a 225 EFI I will admit it fowls plugs pretty damn quick and that is not good on the power head. I can hear it when it is getting close to time to get it done. The only maintenance I do annually is Water impeller and Fuel Filter. Sometimes more often with the fuel filter if I feel I might have had poor grade gas. I have also heard that not letting the motor warm up before going WOT can cause powerheads to go. I pay very close attention to this. This could also be the reason it seems we hear more powerheads blowing during the winter and spring, rather then summer.

Lower unit: Leaking seal letting water in the gear case. Biggest problem from my understanding. I change my gear case oil every 5-6 trips. Watch for water, its not uncommon to see small amounts of water. You will know when it is to much. A few things that can make a seal leak. Normal wear, fishing line and not keeping the boat in the water on pad. I pull the prop off the boat almost right after or right before every trip to the delta or clear lake, looking for line or anything else that doesn't belong. Not as often on the big clear reservoirs.

Some might think the above is all overkill, the motor seems to run really well when overmaintained. bottom line take it easy on the motor and pay attention to the little things.