Design is a fragile thing. One change in the wrong direction and whole machine might stop working properly hurting user experience immediately.

On the outside though design seems to be easy (in contrary to enginering). Anybody can do it right? Let's make all the decisions together: engineers, project managers, designers... and we will see what will happen. Wrong. Good design is always based on specialized knowledge about human behaviour. 

It's designers responsibility to stand up and fight for proper argumentation and proper decisions. No matter who's making a decision and who's trying to mess with your design, you're oblige to present best arguments for, or against certain case. Best arguments are using facts not opinions.

Fortunately for whole User Experience community we have whole bunch of methodes that we can use to convince our teams and stakeholders to do the right thing. Here's a little graphic presenting methodes that I use (or I'd like to use). Hope you'll enjoy it.