In general, always main 2 champs per role you're trying to RANK with, so you learn and understand the matchups.
Also use every tool available to you (camp timers, or scouting to see which enemy player could be on tilt/longest losing streak etc.) As a SUP or JG go camp that lane.
As a JG its a Time Race.
Look at team comps, choose which lane character has a better chance of carrying the lane and get that lane ahead (or prioritize pathing to optimize gank patterns in favor of that lane).
As a Sup, similar to jg look at what lane you want to assist more (ie Yummy sitting on fed Garen or WW etc.).
Early game calculate if your lane wins lv1,2,3 matchup and try to bait an all in to define the lane, and then just deep ward to keep track of enemy jg to optimize counter play or rotation.
And lastly cant stress this enough, if you're playing rank and you lose 2 in a row TAKE-A-BREAK!!!!