I get this question at least once a week.
While the answer is a simple one, it is not simple enough.
I wish I could always say "breathe out on the way up", but I can't.
The real answer is this....
"If you are going to breathe out, do so during the concentric portion of the movement."*
What is the concentric portion of a movement?
To keep it simple, the concentric portion of a movement is when you are overcoming a load.
Simple examples of this are the upward motions used during the bench press, squat, pull-up, and bicep curl.
On the flip side, it would also include the downward motion of something like a rope tricep extension or a lat-pulldown.
It could also be the lateral motion towards you during something like a seated row.
As you can see, there are several directions in which you can overcome a load (aka concentric motions); and as a result, there are different times that you should be breathing out.
To summarize ... "Breathe Out When You Are Overcoming A Load".
When should you breathe in?
Take a wild guess ...
*Note: I am a huge fan of using the valsalva maneuver during training, which actually calls for the athlete to hold his or her breath during the movement.
This is an advanced technique that I will not address in this article and should not be used without the direction of a high level coach.