If you haven't used the dog wash before, bringing a towel is a good idea in case your dog doesn't like the blower. A towel is quite useful for long haired dogs as they can take a long time to dry with just the blower. If you happen to forget a towel, just let an attendant know, they'll be happy to lend one to you.
Avoid getting water inside your dogs ears as their ear canal is quite long and can trap water easily. Also do not point the blower directly at your dogs ear canals, especially at close range.
Bring a some treats to give to your dog to reward them and create a more positive association with being washed.
Try not to spray your dog in the face with water as it usually causes them to 'shake off', wet your hand and wipe your dogs face to clean it.
If you have multiple dogs, the wash tub can accommodate two medium sized dogs at a time, or even three small dogs!