This has got to be the most important rule of training jiu-jitsu; you must respect the tap.
The reason jiu-jitsu works is that the training involves a large amount of full resistance training. This means you learn to use the techniques under proper pressure, not just in a sequence of pre-determined moves. But in order to do this, and to be able to apply submissions with proper intent, you have to have a way to signal you have been caught in a submission... and this is obviously the tap.
So as everyone who trains jiu-jitsu knows, the tap is normally the act of tapping your hand on your opponent multiple times to signal you can't escape and would either go unconscious or have a joint broken by the hold. But a tap can take other forms... most of the time you tap on your partner's body but sometimes it will be on the mat, sometimes you will have to use a foot if your hands and trapped, and sometimes you will have to verbally tap. To verbally tap you should always say "Tap" and nothing else unless you know your partner definitely understands what you mean.
What follows from this is that there are better and worse ways to tap. Personally I think verbally tapping is the best option; it allows for a much higher intensity of attack, especially between higher level fighters. After this is the most common method of tapping with your hand on your opponent's body; for this you should always tap multiple times and fairly hard. Then if you need to tap the mat (due to positioning) you should, again, tap multiple times and hard enough to make a loud sound. If you are using your foot, it will pretty much always be on the mat, so make sure it is repeated and loud otherwise your partner may think you are just trying to base.
Now, the most important part of tapping is that it has to stop the hold being applied. There is no excuse for not releasing something immediately in training. If you break this trust then the rules of sparring change and it is no longer a safe environment. This is obvious to pretty much everyone who trains. However, what some people may not consider is the other side of it... the person tapping also has to respect the tap; you should only tap if you have to. Just because you are stuck in a position it doesn't mean you can tap in order to restart. You also shouldn't tap before someone gets close to finishing a submission; they might not even want to submit you with it and could be waiting for you to escape so they can try countering it. If you tap too early you cheat your partner out of proper training experience and you also completely avoid learning to escape in a full resistance situation. So this means the respect between sparring partners is balanced... you know you have to stop when someone taps because they will only tap when they have to.
The last part of this should be obvious, but over the years I have seen most of these rules broken. There are no circumstances where it is ok to fake a tap. Making a hand movement seem a bit like a tap to try to get someone to release and then continuing to fight is not only cowardly but it will quickly turn a training environment sour. A tap is not a slap either, and should never be aimed at vulnerable parts of the body (although this could happen accidentally); you might be annoyed (sort your ego out!) but if you think you are getting some sort of petty revenge you should remember you could have been put unconscious or had a joint destroyed! Also, as I've already covered, you should only tap because you have to. This doesn't always mean a submission, you may feel like you're about to be sick or feint, or may have injured yourself during a movement, but never, ever tap because you're tired or unable to escape. Again, doing this will make your partner start to not respect your taps... everyone knows the story of the boy who cried wolf (if you don't, Google knows).
Spar hard, tap if you have to.