Habits
Play crossfires
In one line: With a teammate on the same site, watch the same entrance from two different angles.
Why
Whoever comes in can only check one angle at a time. If the two of you cover the same entrance from different sides, whoever fights one of you gets shot by the other.
How to train it
At the start of the round agree on it: "I watch main from here, you from there". Try two or three combinations and keep the one that works.