Well, I didn't have much success in my stint working at a gym (partially due to limited availability on my part [I could only get there by public transport, which took a whole third of my work potential], partially due to the meetings on sales skills bearing little semblence to reality [they were busy teaching us selling spiels rather than the overarching principle of how to rhetorically communicate the value of PT], partially due to being taught how to close a sale but not how to open in the first place [obviously, if the sale never opens, there's no opportunity to close], partially due to favouritism [the fitness director liked some trainers and practically force-fed them clients; didn't like me and kept my shifts empty -- our main basis for sales was in member inductions, and I'd often have a shift at the same time as another PT, theirs would have 4 inductions in it, mine would be blank, and I'd often go a month at a time without anything scheduled into my shifts], partially due to factors that I'm probably still unaware of, and partially due to dumb luck). So, I can only go by what others who have had success say, while relating back to the causes of my failures.
So, first thing I feel should be mentioned is demographics and statistics. On average, about 5% of gym members will pay for personal training. The rest do not see the value in it, and cannot be easily convicted of new-found value in PT. Depending on the individual gym, this ratio may be higher or lower, but 5% (or 1 in 20 members) is a fairly reliable average.
If you're in a gym of 3,000 members, that means there are roughly 150 members who will pay for PT. Does that sound good? According to the fitness director/PT manager who signed you up, it probably does. But factor in that there are other PTs working there who already have most of those 150 clients, and the odds aren't so wonderful. There probably are enough members for you to establish good business if the right circumstances are met, but if your goal is to get to the point where you have a customer waiting list (which is pretty much the ideal situation), the stars are really going to have to align right to get you there. As you saw in my opening paragraph, for me the stars aligned almost as poorly as possible. The good (probably) news is that if you have the availability to work whenever the gym's open, and you know how to open a sale, you're already halfway there.
Of the members who are likely to become PT clients, most of them want to lose weight (probably 90% of clients), and the majority of them are probably women (maybe not 90%, but almost certainly more than 50%). Depending on whether your gym is in the city or in the suburbs, they're either career women who insist they have no time to spare (so you'd expect to be doing 30min sessions before normal working hours, during lunch breaks and when people finish work), or they're housewives/stay-at-home mums who will probably do longer sessions, but pay less per hour (in the CBD you can get $50/30min as a going rate and often higher rates than that, in the suburbs you'll probably be doing 45min for that some price, in lower socio-economic places it might be $50/hr). So, being able to behave in a way that's engaging, friendly and professional with women will get you a long way.
Of course you definitely can target other demographics, but 25-40yo women who want to lose weight seem to make up the lion's share of potential clients in most commercial gyms. I tried targeting other markets for the sake of the relationship I was in (even though I'd never do anything unprofessional with a female client, my girlfriend at the time was not okay with it for obvious reasons)...this was probably also a contributing factor towards me not having enough clients to survive. So I feel that if you want to target other demographics, you're probably going to have to really excel and stand out as THE person to go to for that market. In the 25-40yo female market, you can get away with being A person to go to, but outside that market, I wouldn't bank on anything less than being THE person to go to.
On the note of being THE person, consider specialisations. Even if you want to go with the major market demographic, you still want to stand out as distinct from the other PT's. Every PT in the gym claims to specialise in weight loss, hypertrophy and strength+conditioning. A lot of them don't know anything about any of those things, other than that weight loss has something to do with calories, hypertrophy has something to do with muscles and s+c has something to do with being faster than a snail. So sticking the same list of things on your profile doesn't really set you apart. When I graduated with my Diploma of Fitness, I technically specialised in a page-long list of things. It turns out that after 3 or 4 points, people start to zone out, and instead of thinking that you're really good at everything, they think you're a D&D bard in real life: the bard can do anything but sucks at everything.
Now if I were to do up my PT profile again, I'd probably only say that I specialise in: Fitness For Beginners; Periodization; Barbell Training; and Body Composition Management. What does that equat to? Losing weight, hypertrophy, str+con and being able to teach all of the above to the untrained folks who are most likely to become clients. In other words, the exact same thing as every other trainer claims to specialise in, but just by phrasing it differently I've distinguished myself. What's your training background? What things are you most passionate about and interested in WRT fitness? These are the things that you'll tend to study outside of what's required for your qualifications, and these are the things that will tend to become your honest areas of specialisation.
So, that's target market and specialisations covered. What about actually turning members into clients? Again, I didn't do a great job at this, but I'm told that the key ingrediants here are establishing trust and rapport, and demonstrating competence. These are the things that will create the perception of value, but you'll still have to ask people to be your client once the above is established.
A lot of PTs are so caught up on being gun salesmen that they'll basically trick people into becoming clients through swift sales speeches that make people feel like they have to become clients. This may get a PT pack sold, but it doesn't make the client trust you or like you, and they'll be quick to find excuses not to train, then stop training altogether. If you go this route, you'll get a lot of people in the door, but they'll be walking out the door just as quickly. The few clients I've had have been longterm clients, all lasting several months (I've had one for the last 9 months, I had another for 6 or 7 months who only stopped training with me because I stopped working at that gym). I suck at getting them in the door, but once they're in, they're usually happy to stay because of the trust and rapport established and competence demonstrated.
On the other end of the spectrum are trainers who are at the gym all the time, but they're there on facebook instead of out on the gym floor working with people, demonstrating competence, and establishing trust and rapport. If they get clients, it's not because of anything they're doing in the gym to get them. They might be getting fed clients by the manager, be sourcing them from outside the gym, or otherwise have dumb luck to get their businesses going....or they have no business going for them, because they're not doing anything to create business. As a general rule, it would be wise not to be that guy.