Efficiency is a function of the available HP minus the frictional and other loses that rob that available HP from the prop.
They include these small easy to do items.
1- Use friction modifiers in all oil
areas like the gas & oil
if it's a 2 stroke
2- Use friciton modifiers and synthetic oil in the thinnest that's acceptable to use in your environmental conditions.
3- Best gap on plug(s) for the tune-up.
4- Clean all debris and buildup off the lower unit that sees the water
flow around it.
5- Use the best gas you can as far as octane rating goes and then take adbantage of it and advance the ignition timing "slightly".
6- Make sure the vessel it's pushing thru the water
is balanced and loaded properly and the bottom is clean and you're not dragging anything to slow you down.
7- Make sure the air cleaner is as clean as it can be and the proper size without restrictions to air flow.
and for those seeking to increase efficiency during an overhaul
1- As stated above, port & polish to the head
2- All of the above.
There's a lot more things I'm sure people will post but these are the ones that come to mind immediately for me.