For me if I’m not careful my mind will wonder in the gym a bit so I try to do things like focus only on my workout, not scrolling through my phone, not the hot chick on the leg machine or that another guy could possibly have bigger arms than me, I look straight ahead but don’t look at anything and do slow controlled reps and concentrate on squeezing the muscle and after 30 years I’m still improving on how to work the muscle I’m actually working on