How to Choose a Dog Trainer

Perhaps the best analogy for choosing a dog trainer is that choosing a dog trainer is very much like choosing a therapist. I can say this, because I work in mental health and know therapists well. Just like a therapist, it is really hard to lie to a dog trainer! "Did you do your homework?" "Yep." Dog runs around, ignores, does anything but what they should have practiced. "Did you really do your homework?" Head hung in shame, "no." They see how frustrated you may be by your dog's behavior, and how vulnerable and embarrassed. This is a person you are trusting to work with a valued member of the family! You may get called out for mistakes you've made. You may feel defensive, but the thing about being defensive is we usually know the other person is right. We don't want to admit it, but usually they are. You may leave a training session feeling drained, physically and emotionally. Your dog trainer is someone who you feel you are willing to take their advice, celebrate successes with and trust that they are helping you, no matter how you are feeling in the moment. That is why, it is a good idea to find a trainer you mesh with. 
So, here's what you do: Google dog trainers in your area and check out some websites. Have an idea of what you want, whether it be group classes, one to one sessions or board and train. Do you know what training method you would like to use? A trainer should be clear about what their method is. The ones that you like, call them or send an email, see if they are willing to take you and your dog on. Be honest about what you need. Meet them, see if you can observe a class or a consultation. If you like this person, hopefully you have met your trainer! 
If a trainer says they are unable to work with you, it probably is nothing personal. My insurance, for example, doesn't cover wolf/dog crosses in any way and I am not to train dogs in protection sports, bite work or for police and military. Maybe the trainer only does a specialized training service, such as training for sport or task. The trainer might not be comfortable in working with aggression, and this is actually to your advantage. You want your trainer to be confident in what they are doing, and a trainer who admits they don't know it all are less likely to mess up by taking on something they can't do. 
The trainer will probably recommend a training package, if you are going with one to one sessions. This is because no dog is going to suddenly learn to walk at an obedience ring heel out and about after one session. Plus, even if they did, are you going to remember all of the skills next week? Multiple sessions are for yours and the dog's benefit. Plus, this gives the trainer time to add in other things to teach you if needed. 
Please don't choose a trainer based solely on cost. Trainers put a lot of time (and money) into getting their education and experience. They may have done an online program or an in-person internship and perhaps have taken tests to prove they know what they are doing. They attend workshops and seminars, read all they can, participate in live webinars and groups. For some certifications, trainers are required to earn Continuing Education Units (CEU's) just like school teachers! None of this is free. The trainers I know, myself included, have gone into this industry because they want to help dogs and their people. That is it. But, they have to make a living as well. However, no one says, "Hmm, dog trainers have an easy job and are often on the Forbes 500 list. I'll be a dog trainer." 
So, how important are those certifications that trainers are earning? A little background: dog training in the US is largely unregulated as an industry. Any person can call themselves a certified dog trainer, just because they had a dog at one point. No, even if not, they still can. Outside of veterinary medicine, this is how the animal industry is. However, there are trainers who take the field seriously and are getting certifications that are to a high standard! If a trainer can't tell you where that certification is from or you can't find it multiple places on their website, good chance they are pulling one over on you. Are there good trainers who have no certification? Absolutely, yes. It does come down to experience, but those learning opportunities help trainers be more efficient and understand what is happening during sessions. There are probably also some not so great trainers who passed tests with flying colors. 
How do you pick a trainer? You pick someone you connect with. You pick someone who can help your dog. If they see you and and your dog at your worst, they are going to celebrate with you when you are at your best!