Crate training or any type of training for that matter is not something that puppies necessarily take to better than adult dogs. In reality dogs are dogs, but I think the impression that adult dogs are un-trainable comes from the fact that some adult dogs have already learned wrong behaviors that need to be slowly unlearned before training can start. The advice will be relatively the same regardless of the age of the dog.
Some important things to note:
Associate Positive Emotions and Rewards
Dogs memories are largely based on emotions. A particular activity or situation evokes either a positive emotional reaction or a negative emotion like fear, with the dog having no ability to rationalize this. The crate as a location needs to be associated with positive happy emotions.
First and foremost ensure that the crate is sized right for the dog, they need at least enough room to stretch their legs and turn around.
Give them a comfortable bedding or cushion that they can sleep on. When they are alone in the quiet empty house in an enclosed space with nothing to do, they will sleep.
When training, start small. Using a treat, lure them into the crate just a bit, give them a lick when they get a foot or two inside. Repeat this ad nauseum until the dog starts initiating the desired action on its own with hopes of a treat. Gradually lure him in more and more repeating until he is comfortable. Only provide the treat when the dog looks completely calm and comfortable inside the crate. Once the dog is completely inside, close the door and wait for a bit. If the dog remains calm for a short amount of time then provide the reward and start slowly increasing the time with the door closed.
The final step here is to close the door and leave the room very briefly. Return quickly and reward the dog if they were calm. If they start to panic then take a step back because maybe they are not ready for this step just yet. Slowly start increasing the time where you have left the room until you can leave the dog alone in the crate for at least 5 minutes with the dog remaining calm and relaxed the entire time. At this point you and the dog will probably tire, so take the rest of the day off and try it again tomorrow without treats but with affection. The dog will probably take to the crate very well at this point.
Do not award an excited state of mind or fear and anxiety
Crates can be a scary thing for a dog that is not used to it. Have patience and slowly work through their fears by forcing them to face their mental block. This is a controversial approach as many dog behaviorists disagree on the most humane way to deal with a dog's fear but this is a very direct way that seems to work extremely well for myself. I believe the best way for a dog to deal with fear is to be desensitized to what causes it fear.
Negative reactions mean that the dog is not quite ready to move to the next step. Take a step back and only reward the dog when it appears calm and relaxed.
Do not EVER use the crate as Punishment!
You do not want negative state of mind to be associated with the crate. Lets face it, dogs can be aggravating little buggers from time to time. You must resist the urge to put the dog in the crate just so he will leave you alone or to punish him with "timeout" for unwanted behavior. This can sabotage crate training.
It is a natural instinct for dogs to have a "den", a comfortable place that is warm and safe, and where they can safely sleep. Dogs have an instinctual aversion to peeing or defecating in their dens. You want to foster the attitude that their crate is their "den". This will mean they will be calm, and less likely to go in their crates while you leave the house.
Watch for Separation Anxiety!
If your dog has issues with separation anxiety then crate training may be counter productive for them. Try to work through these issues first before attempting crate training.
