I'm not a pro, but imo balance is key. If poker is THE most important thing in your life you should reevaluate anyway. Everyone will feel down if he's had a bad day, but it's easier to cope with it if you have another activity that captivates you the same way.
In my case, 1.5-2hrs of Thai boxing does the trick ;)

I've found that poker affects my mood most when I have some kind of expectation for the day (ie 50 games, 1k hands, $X), and that expectation is somehow not met.  Before each session I try to ground myself, aware that variance could be around the corner waiting to steal my monies, but confident that I'm a winning player in the games that I'm about to join.
 
If you can see red at the end of the day and not be disappointed, you're inhuman.  The tough part is trying to compartmentalize your poker play from the rest of your life, but IMO it's not teachable, it's just something that you're going to have to figure out for yourself.  I took 2 months off from poker because I felt that I wasn't mentally prepared to deal with its swings, and coming back to poker, I feel like I have a stronger perspective on my own mental states.  And if a short break doesn't work, a post-session blunt always will :)

 Quoting Phil Galfond
"Poker is just poker, and money is just money. If you let it become your life then you will base your self worth on your results. That means that you'll base your self worth on something that you have almost no control over. 
Poker is my job, which is a just part of my life, and one of the least important parts at that. "
 
I know it's not spot on, but close enough. Get a hobby, hang out with friends, work out more etc etc and let poker be poker.

I find that the amount that it dictates my mood has a direct relation to how i react the MOMENT I get bad beat. Basically, if I let it get to me for even a second then it is stuck in my head. I try to just focus on that I'm going to play my best game and ignore the results before I even start my session. It doesn't always work, but I think it helps.

Just do what Ivey does and quit real quick when you're down in a session.
Works for me, but then again I don't play much volume.

today I went on a run of 8 losses after I lost my first hand to someone calling me down with 57o
board 89A
I bet the board with the A
And lost to a 6 on the river as he called his whole stack off in the first hand
Put me in a s****y mood for ages
 
Came back tonight
 
First big hand I caught was pair of ten ace
Flop comes
A 10 10
and I lose all in to a pair of aces
leaving me with 150 chips, eventually battled back to win that game, in that game I was some how philosophical about the loss and it didnt get to me
 
Somehow not lettig those bad beats "beat" you is the trick, I know when I am steaming and try to get away, but I find it tough. Primo explained you need to get away when you know you are running bad
 

I quit very quickly, but it does stifle growth.  I don't get in nearly the amount of hours as I'd like to with actual games.
If you can play a session and quit quickly if you're running bad, regroup and play some more an hour or two later, I think that is optimal for a lot of people's mentality.
Of course, many people can't do that and just play through it, losing some edge in the process but playing many more games than they otherwise would bc they can't go back and forth between playing and quitting all day.