Every martial arts school has its' own rules of do's and don'ts. However there are certain things you should observe that make sense anytime you go onto the mat to train regardless of where it is. I'll discuss a few of these in the following paragraphs.
Be clean. You should always go to the gym clean. No one wants to train with someone who stinks. Dirty feet is also pretty disgusting. Admit it, you don't particularly enjoy training with someone that has body odor. Well, in like manner, no one enjoys your body odor either. Be considerate enough of others to make sure you are clean when you begin your training session.
Wear a clean uniform. This goes with the first tip of having a clean body. Your uniform doesn't have to be new or even ironed/pressed, but it can and should be clean.
Keep your fingernails and toenails trimmed. This helps avoid nasty scratches on your training partners. I've seen numerous eye injuries from fingernails and toenails that were too long.
Never wear jewelry; especially necklaces. Jewelry can cut and damage others. It can also cause injury to the wearer. Necklaces especially can and have cut throats. One incident at a local tournament with a necklace caused a severe throat injury that could have killed the woman wearing it had a doctor not been present. Jewelry can also be broken easily. Since most jewelry is expensive it only makes sense to protect it by removing it during training.
Always show proper respect to your instructors and fellow students. This is a big deal in martial arts.
Be attentive. Pay attention to what your teacher is saying. Pay particular attention to the details. Leave your playing and silliness outside the gym. Martial arts is a discipline art. It is also a contact art. Playing around, being silly and not paying attention can get you or others hurt.
Practice at home. Martial arts have to happen automatically. If you have to think about what to do it is already too late. The only way to get to the point of an automatic response is through regular practice. If you just practice when you go to the gym a couple of nights a week you won't develop your skills to where they need to be when and if you ever need to actually defend yourself or your loved ones. It is better to sweat a lot from practice than to bleed a lot in battle.
You can read other articles that I've written about martial arts topics on my blog: http://mykarateplace.com