I have a small poochon (poodle/bichon frise) dog that gets excited when I try to put him on a leash. It is nearly impossible for me to put his leash on when he is excited like that. How can I teach him to not get excited when I put his leash on?
Positive reinforcement here is the key to keeping your dog calm. Obviously, dogs find walking extremely exciting and associate the leash with going out and having fun. Working on your dog's basic training commands on a daily basis is vital to calming him, so at least two times a day some basic sit and stay commands with him, always rewarding him when he gets these right, but also start to reward when he is correct and also calm. When the leash is needed, turn putting the leash on into another command session.
Ask him to sit, he should want the treat so will try to carry out this command. If he remains calm, you should be able to put on his leash. Another factor to consider is that your dog needs to learn that he won't go out for a walk without the leash, so try to avoid chasing him about with the leash to put it on. Call him to you by the door when you have the leash, and only put it on when he is calm and responding to commands. This requires time and patience, so try to keep calm yourself and simply ignore any overexcited behaviour, and immediately reward him when he does what you are asking of him.
I encountered this behaviour a lot with dogs in the rescue centres I've worked at, as dogs are so excited to get out of their kennels, so consistency and patience are going to be your best friends. If you are struggling with his behaviour, then the best people to ask would be behaviourists.