Mirrors are not for you to admire yourself, and plates aren't meant for you to strew about. Watch what you do.
Mirrors
There are two things to keep in mind about mirrors in a gymnasium. Firstly, they are there to help make sure you are using the proper technique. Especially during overhead presses, deadlifts, etc, people rely on mirrors to maintain balance and keep track of form.
Secondly, try to avoid walking in front of, or blocking the mirror view of others while they are performing their set; it will interfere with their concentration.
Plates
When you're using a plate-loaded machine, be sure to put back those plates on the rack when you're done using them. Remember, other gymnasium members are not there to participate in a scavenger hunt.
After using a bar, leave it completely free of any weight plates. Don't assume that everyone can lift the same amount of weight you can.
Drop
Do not drop weights from a height, specially during free weights exercise. You certainly don't want to jeopardise the safety of fellow members by having plates fall out, or dumbbells rolling around.
Also read: Stability ball: Have fun, lose weight