When writing code with unit tests, I find it helpful to view the tests and the implementations side by side. I use vertical splits in Vim to do this.
The stupid brain trick is this: Put your tests in the left-hand split!
Why? I traditionally used the left-hand split for the "main" thing I'm working on and the right-hand split for reference. I've watched many other developers do the same thing. The right split is constantly sub-split horizontally and always changing to various different files.
In trying to be better about writing tests first, I created a test code file before the implementation file, so it just happened to be in my left-hand split. After an hour or so of development, I realized that the "main" file placement of the tests meant that I focused more on them. I viewed the actual implementation file as just some ancillary file I had to tweak to get my tests to pass. It changed the focus of what I was doing from "Write a method that does X" to "Describe X". Describing something involves breaking it up in to smaller and smaller pieces until the implementations are so trivial that they basically write themselves.