Not much of a writer really..

But since Yuumi have gotten such a **** reputation I feel like something have to be done as she is like the best sup ever x)

Simply:

Dont stay attached at all times..use aa as mush as you can, not just for shield, Yuumi is made for keep poking until its blasting time, and when that time comes her ult makes it pretty hard to escape when used right.

Ask your ADC to run closer to the enemies when you use Q while attached, tell them to be a little patient in the beginning, and before you are to use your ult after lvl 6, make sure the target have no more than 50% hp, and "ping target" the lucky one in order to coordinate the attack with your adc (Sucks if your adc suddenly runs the other way right when you ult, giving the enemy a chance to not be hit with 3 waves which will root them)

if you are pushing to their tower, always have a ward in the bush at the jungle entrance, and every time you shoot an enemy while they are under tower, jump off and move away from tower damage quickly and jump back to adc (and jump off and aa anytime you can if there are minions between you and the enemies, tho be super careful if they got any sup champs with stuns or hooks)

Late game: stay with whom ever is carrying the game, and in team fights try to find a balance between jumping to whomever is the most targeted champ for healing, and whomever is closest to the enemy group being the most potential target in order save them with ms boost and heals if they need to get out.

Also just in general, do NOT go into the jungle alone, ever.. that is too much of a gamble. When your adc or jungler moves trough the jungle, jump on em and ward all the way!

ooh and one last thing, even if the champ you are on is in close combat, when using Q, let it travel for a sec if you can and do not shoot it right on target, this can save your host with slowing the enemy down, and it does close to double damage.

Sorry for being a **** writer :P