Location:
Location is crucial to ensure regularity and consistency in workouts. If possible, join a club within walking distance from work or a short drive from home. A simple tip to trigger one's guilty conscious is to select a gym that is often passed by during daily errands. This will remind you to lace up your sneakers and start sweating it out. Prime locations will save time and provide the extra motivation needed to hit the gym regularly.
Staff:
Especially when first starting an exercise program, the best way to judge whether a fitness club is adequate is to ask about the qualifications of its fitness trainers. Trainers with huge muscles may not necessarily be qualified for the task. Fancy fitness certifications can simply be purchased from popular magazines. Ask basic questions on the instructors' certifications and educational backgrounds. It is also recommended to ask existent customers what they think of the gym and the trainers. It is one thing for the instructors to be fitness experts, but they should also be available without always expecting compensation for their advice
Hours:
Make sure the gym's hours coincide with your availability. People who'd like to train early in the morning might have some services restricted, and those wanting to train right after work might have to contend with long waits for use of popular equipment. A simple tip is to visit the club when you expect to workout and observe how many people train and how many machines are available. Take notice of how long people wait for weights and cardio vascular equipment that you expect to use. For some, this might even be a helpful experience to schedule effective training periods and avoid hectic peak times.
The People:
Every club caters to a certain type of clientele so make sure you feel comfortable with the members. Obviously if everybody around is extremely buff and screaming all the time, you might feel uncomfortable working out in such an environment. In general, people feel comfortable training beside men and women of similar ages and interests. So the best way to get a feel for who you will be sharing the workout space, is to visit the gym at hours when you will be training.
-^From Askmen.com ...
The main thing for me though is choosing a gym that a) has proper equipment and b) is not over crowded. Theres nothing that ticks me off more than a ton of people getting in my way, taking up machines, etc.