Put on a higher weight than you can safely lift to not be outdone by the guy next to you, especially if your bro is watching.
Grunt/scream really loudly and slam the weights down, regardless of if your doing your max or just warming up.
Don't spot exercise your glutes, never stretch or train mobility or balance or cardio.
Don't wipe the equipment after use.
Don't take the plates off of the machines afterwards.
Do spill pwo and protein powder and don't clean it.
Use deodorant or body spray instead of showering after ("I already showered this morning").
Insist on jumping in between sets when someone else is working out, especially if it's a woman.
Offer (mostly poor or extremely basic) advice on technique to everyone who uses lighter weights than you, regardless of their actual form - especially if it's a woman.
Be insistent than any woman with headphones needs to take them off and pause their workout to hear the most pointless question or comment you can think of ("where did you get that?" & point to a bag clearly printed with a chain store name, or "it's good you keep your hair in a pony tail, don't want it to get stuck in the machine!", or "why do women always do that leg-kick-excercise? You don't need it, I like small asses anyway", or "have you tried this gainzzZ-drink?(hold up a half-drunk ready-made shake), it's not too chalky compared to their previous recipe").