Where do these cretins get the idea that it is OK to ruin everybody else's experience of a performance due to their own ignorance of those around them?
Small children: if you haven't trained your child(ren) to act responsibly at a public performance, don't bring them! If you do bring them, and they start to act up, be considerate of every one else and leave! It may surprise you that others don't pay for tickets to performances to experience the abysmal behaviour of your family. If you choose to stay, at the end of the performance offer to pay for the tickets of everybody else at the venue in compensation.
Mobile phones: Turn it off. It's that simple. You are not important enough to leave it on. Really, you aren't. And any call that comes in isn't a critical call. If you are waiting on a critical call, don't go out to a public performance. If you are a person who has a responsibility in society to respond to emergency requests (doctor, fire dept director, etc), you should be clever enough to know how to put your phone in silent mode. Persons in society who are specifically not holding that responsibility are businessman, politicians, and social butterflies. If you must ignore this, just offer compensation to pay for the tickets of everybody else at the venue. And giving some money to the cast would be gracious too. An important person like you can certainly afford to compensate everybody else.
Illness: If you are sick, stay home, get well and give your ticket to someone who isn't ill. Don't go out, infecting large crowds with your disease, coughing and spluttering your heart out.
Wrappers: Ladies, you know who you are. You decided to bring along that cough lollie (because you were ill), or discount candy (because you are too cheap to buy something at the candy counter). At that really quiet, emotional point of the performance you decide to open that wrapper, and in consideration of everyone else, you do it as slowly and loudly as possible so nobody will notice. Why is it I am always the one sitting in front of you? Did you ever notice that almost all of the packaging available of the items at the candy counter don't make a great deal of noise when opened? No, you didn't. If you must bring something along, use a reusable cloth container, and bring food items that aren't individually wrapped and are quiet to munch. Your neighbors, your budget, and the environment will benefit.
Personal Hygiene: If you can't be bothered to bathe, or wash your clothes properly, you shouldn't go out. Nobody want's to see you in an unclean state. This includes using public transport. It doesn't matter if your apparel has been out of fashion for 10 years, but it does if it isn't clean.
Sitting Properly: Sure, you can put your feet on the seat in front of you, as long as you pay for it, clean it after, and give some compensatory dry-cleaning funds to the persons on either side.
Litter: Nothing gives you the right to drop your cigarette butt, candy wrapper, bottle cap, straw, bottle, bag, receipt, or other item. Dispose of it properly.
Talking During the Performance: Do I really need to go into this?
Alternatives: If you can't abide by the simple, basic rules of society, don't go to a public performance, go to the football instead. At the football you can possibly engage vigorously with the other persons of a like mind, possibly reducing your impact on the gene pool.