Next month, gym crowds will be at their peak due to the invasion of millions of New Year's resolution-aries. Many of these newbies will hire trainers for help, and consequently, many of them won't see results and quit. I've worked with over 100 trainers in my five years at MF, and have been impressed by very few. The following are some of my recommendations for determining what kind of trainer to hire or listen to.

Make sure he/she has experience. There's hardly anyone who writes for MF who hasn't been training people for at least 10 years. All the education, certifications, and gut instinct in the world can't substitute for having spent years in the trenches of fitness. Our training adviser, Jason Ferruggia, once told me, "A trainer shouldn't write an article until he's been working for at least five years. He shouldn't write a book until he's worked for at least 10." As with anything in life, the more you do it, the better you get. The more you understand. The sooner you can see the game from three moves ahead.
I've met far too many trainers who base their philosophies on the methods that worked for them personally, or on a handful of clients. Show me something that you've seen work on 500 clients of varying body types and levels of experience, and then we'll talk. 
Don't judge the trainer by how he/she looks. I absolutely believe a trainer should walk the walk. You wouldn't take financial advice from a firm that consistently loses money (although that seems to have been the case for a lot of people as of late), so it only makes sense that your trainer should be in reasonably good shape himself and know what he's doing in the gym. But just because a guy is 225 pounds and ripped doesn't mean he's an authority on how you should train. Some people are just gifted genetically. Some people take steroids. Others have just learned what works for them. Everybody is different, and his program probably shouldn't be your program.
I've known a lot of great trainers who didn't exactly look like they were carved out of stone. Most of them did at one time, before wives, kids, long hours, or illness and injuries took their toll--but as long as they weren't grossly out of shape standing before me, I was willing to listen to them. More important than if your trainer is in shape is if his/her clients are. Did the trainer help a guy lose 50 pounds last year (and does the client look and feel better because of it)? Does he/she have clients who have the kind of size and definition that you're going for? Those should be your questions.
Know that less is more. If your trainer hands you a program that has you running around the gym for an hour or more, politely excuse yourself and find someone else. There's no reason your workouts--regardless of your goals--should last more than one hour. If the trainer wants to stand over you cursing, blowing whistles, and pushing you beyond the limit by physically helping you force out reps, that trainer sucks. Yes, you do have to train with intensity to see results, but every workout shouldn't be World War III. Some trainers feel like their clients will think they're getting better results if they beat the crap out of them in a session, but it doesn't work. It will make you hate exercise, and it will get you injured.
Don't be cheap. I know trainers that intentionally charge more than their competitors in order to discourage apathetic clients. Not only do they not want to work with people who aren't motivated to succeed, but they're that good, and they can get away with it. You can save a lot of money by hiring a trainer that gives you free sessions and other kinds of bonuses and goodies. But if that's what he needs to do to drum up business, he's not worth it. Paying more for a trainer increases the chance you'll get somebody good, and it will make YOU feel more of a need to get your money's worth.
Since your own level of motivation is the greatest factor in achieving your goals anyway, it makes that much more sense.
Happy holidays to everyone who reads this blog. And to those who don't... please start.