Always use good oil like Mercury Premium Plus or Evinrude XD 50, Yamaha and Amsoil would be good also.
I put Startron in my fuel, because it could sit for weeks between use ....
http://www.starbrite.com/en/startron?gclid=CNSL7oGi7cACFRMLMgod-jcAkgAlways let your motor warm up before giving it full throttle.
Squirt "Deep Creep" (Seafoam in a spray can) into cylinders at the end of season or start of season, heck middle of the season if ya like ... Gets rid of some of the carbon deposits on pistons and around ring lands, plus cleans out exhaust system.
BIG plus is the HUGE white cloud of smoke coming from your motor after starting with deep creep ... LOL
Running the right prop is a very good idea, keeps your motor in it recommend RPM range.
Having two props is also a good idea, one for speed and one for pulling skiers/tubers.
Replace all old or leaky fuel hoses in your fuel system with the newer ethanol resistance hoses.
If hoses/fuel lines look OK, wipe them down to remove any fuel / oil / dirt, run motor out on the lake or on water muffs.
Check to see if any of the hoses are wet with fuel, if so replace it, it's most likely bad or going bad.