Man you’ll be keeping your energy low without some order in selecting tracks… Anybody who say that’s wrong knows little about mixing IMHO.
My point to that was that don’t just say track selection, if you have any tips for actually mixing then post that. Track selection is always a given.
And I didn’t…
I could speak of techniques and share tips on mixing too but here was word on keeping the energy level constant and nobody said anything about how track selection can be your good friend which helps you not to break the flow. Sorry if that wasn’t too obvious for ya 
One way of keeping energy in a set is to move up in key from tune to tune, that way you get a build in energy even if you are not smashing out bangers, you can then move down a few keys sequentially before bringing them back up again - if you jump a couple of keys then the energy builds faster…
One thing i would say is know your music, if you know theres a breakdown coming that you dont want to ruin the flow then throw a loop on and mix into that or use that loop as a build up and drop it in! This is what tend to do.
Skip breakdowns and such by just skipping ahead. Traktor stays in time with the beat.
Ive been messing around with doing this and you can keep the music going pretty well. I only will do this for certain songs that have huge long breakdowns and a groove thats good enough to keep going for an extra 32-64 bars.