Controlled chaos is good for proofing and having fun too. This doesn't mean you should have 3 of your own dogs running loose, all trying to please you at the same time. Generally this leads to you screaming at one and affecting all. Remember, our dogs understand body language and tone of voice, they don't ACTUALLY understand English. If you don't believe me, ask my friend that accidentally taught her German Shepherd to lie down every time she said Damnit! Even in craziness your dog should have your near sole attention and devotion. I say near, because just like driving, Agility generally requires you to at least be a little bit aware. And when I say a little bit, I mean A LOT. While I will admit watching two people send their dogs into the weaves from opposite sides could be amusing, it is a lot more detrimental than anything else. Do that three or four times and instead of worrying about finding the next gate in the weave poles, Fido is going to be a lot more concerned about impending possibility of head-on collisions. So be aware. Of dogs, people, cars, equipment, squirrels, geese and raining meatballs.
Okay lets get to the important part though... rewarding.
Failing is training. If you haven't failed you haven't trained. It is important to push the boundaries and until you have not pushed it a little bit too far, you don't know where your limits are. Failing is also the difference in teaching our dogs to think and have some understanding of what they are doing and just teaching another parrot. But at no time does this mean you should be holding your dog hostage to a reward. Just because your dog 'failed', you shouldn't be bodily removing the reward from them, while flinging insults like a bully in the school yard.
A reward and a marker are two different things. The reward is what the dog gets after you have marked it. So in the dog's mind the process should be something like this: 'La-la-la, okay lets try to lie down, okay nothing, must be something different, let's try to sit, OOOOOOH marker, yeah baby, NAILED it, now where is that damn squeaky rabbit'. You can mark using a clicker, your voice or a French horn for all I care, but you mark immediately and then REWARD. It is always rather amusing when I teach a class and a student comes sauntering over at leisure after doing a 15 jump sequence, just to be startled by my Boot Camp persona yelling at them to 'reward, reward, reward'. Before listening to other people's input, before going to have a sip of coffee you really do need to reward. Even in a class environment, well actually especially in a class environment. Make sure you have engaged your dog in some fashion (with food, toys, cuddles), before listening to your instructor's analysis. Honestly I would rather see handlers 'accidentally' reward (so reward to early and the dog actually made a mistake), than NOT reward immediately after the dog has done something correctly.
Pick your battles. If you are struggling with three different sections of a 16 obstacle sequence and by some miracle, you happen to get one of these sections right. Stop and REWARD immediately. Don't be a hero and immediately try and run the rest. Chances are (since you were already struggling) that you will get it wrong again at which point you are left with two options: Reward and your dog is most likely to connect it to what he last did, which was get a subsequent sequence wrong, or don't reward, leaving Fido confused as to what exactly was right and wrong.
Be in control. People have their preferences, dogs have theirs, but sometimes people are just lazy. You have to know the value, potential, limitations and implications of each reward you use. For one, there are only a few instances where I choose to reward with a ball thrown ahead. This is a very dis-connected reward, its value is higher further away from me (otherwise the fetch game wouldn't have much of a challenge now would it) and once it has left your hand, you don't have much control over it. It is good for situations (especially with the lower confidence dog) where you want to teach dogs acceleration away from the handler. Tug toys are in complete control of handlers, which is fantastic. It should not be used to encourage 'velcro-ism'. Dogs should only come into their tug toy reward once they have heard their marker. The same goes for food. Don't leave the reward lying on the ground if you are going to have control issues. If your dog will shoot off in the middle of an exercise to go and find that reward. If you were a moron and you DID do this, do not under any circumstances scream at the dog for going to get his reward. You can't punish your dog for your stupidity and you do not want to connect any negativity to what is supposed to be your dog's reward. Phone a Friend. Ask someone else to help out and hold the reward and give it at the right moment.
Know your customer. Dogs have different tolerances. Some dogs will completely break down and phone the suicide hotline if a reward is withheld more than once. With dogs like this, sometimes you need to offer the reward without using a marker, just to keep their confidence up. You are basically saying 'okay you haven't gotten it right yet (no marker), but things will be okay and the world won't end today (have some food/a tug). Other dogs don't need a confidence boost and will be happy to try and try again until kingdom come. My Psycho Slinky for example, bless her nerdy little soul, will NOT take a reward if she feels she hasn't done something right, even when I am begging her to take it. I also know that after a few tries of not getting it right, she will panic. So if we fail a few times at any one thing, I will ask her to do something which she is almost guaranteed to get right (like a tunnel), mark THAT and reward. But heaven help me if I tried a stunt like that with 'prom king' Volt, he would take advantage of me in so many ways it is not even true. Volt was issued with 3 cc's of extra confidence when he was manufactured. Volt KNOWS the world won't end, he doesn't need me to tell him that.
Be effective. If you throw a ball as a reward, Fido kind of starts after it and then veers of 47 degrees North East to go and pee on a tree, your award wasn't effective. It is fair and well to have all this theory about what toy is the best or what kind of treat, but if the reward holds no value for your dog, it is pointless.
Your feedback system is the only guidance your dog has to what is wrong or right. So if they ARE getting something wrong, it is best you look at which part of your feedback is confusing them.
In conclusion: Reward your dogs!