1. Assume they know what the client needs
Not every 35 year old housewife and mother wants to lose weight and tone up, have a consultation process that really gets to the bottom of what the client is looking to achieve, also you should be talking 30% of the time and listening the other 70% of the time, people love to buy but they hate to be sold to.
2. Shoehorn the client into one way
Just because you are a super vegan who trains 6 times a week twice a day, it doesn't mean this is right for the client. This is why its called personal training, it needs to be personalised and you need to take into account their current fitness level, their current diet, their lifestyle, their professional life and their family life. You really need to take a wholistic view
3. Go on about their own workout
Doing this when unprompted can intimidate some clients, and more probably bore the rest, you are training them and they are paying you, keep the focus on your client
4. Go on about the next charity bike ride that they are doing
I was at a business networking event once, paid good money to be there and there was another personal trainer there, while I used the business networking event to ask for leads and referrals, I got quite a few, the other personal trainer told everyone about his super bike ride, didn't get any referrals (or sponsors). Keep the focus on the things that matter
5. Use the words "never" and "always"
Easy way to back yourself in a corner is to use the above words, it should always depend on the situation, the clients goal and what resources you have to hand
6. Ridicule other trainers in the gym
Come on, have a bit of professional standard. How do you think that makes you look if you are talking down another fitness professional. If you are continually talking down someone else, you either want to be them, they are a threat to you or you are attracted to them! Maybe not! But you need to take a look at it if you are constantly talking someone down.
7. Ridicule diets and exercise systems
OK I do this to known and select people but generally it's not good practice. It's best to think that everything has a place somewhere for someone, you can have your opinions, just careful where you air them.