OK you will think this is silly but my T taught me this and it works 99% of the time.

Take a deep breathe in , let it out. As you are taking in the next deep breathe focus on how the air feels like when it is actually going into your lungs, as in feeling your bronchial tubes expand with the air... It takes a bit of work to figure out how it feels to you. So try it before you "need" it.

Anyway I swear by it.