First, talk to your friends. If one of them was recently married, their experience, positive or negative is useful information. If they had a good experience, using them as a reference will go a long way toward insuring you will make a good choice.
Second, choose a tuxedo shop with a convenient location. Proximity to the event will allow the company to deal with any last minute emergencies that might arise.
Third, look for a tuxedo shop that owns their inventory and does their own alterations. This will allow them to service the needs of any late arriving members of your wedding party and have them looking their best. A Father of the Bride coming from out of town does not want to be told that the replacement garment he needs will arrive two hours after the ceremony.
Fourth, consider the breadth of selection being offered. For this most special of occasions you should be able to create a look with which you are comfortable. We live in a time where there is great freedom as to what style tux or attire may be worn for weddings. Your outfit should reflect your image and not some preconceived notion on the part of the tuxedo salesman. With the multitude of coat styles, vest styles, and other accessory options, you don't have to settle for an ensemble with which you are uncomfortable.
Fifth, be sure the company can accommodate your schedule. Often companies have very rigid policies as to the pick up and return of their tuxedos. You should not have to pick up the day before the event and return the day after. Even in the busiest of times the store you choose should have a minimum of a five-day rental to accommodate your schedule.
Sixth, be certain that the company has a tried and true system for dealing with members of your party who are coming from great distances. This is always a source of stress for both the customer and the supplier. To avoid any problems, you want to be certain that those coming from out of town have sent accurate measurements well in advance of your wedding. Four to six weeks is a reasonable time frame for the adult members of your party. The exception to this rule would be any small children participating as a ring bearer or junior usher. Measuring them much before three weeks prior to the wedding may not be reliable as they grow so fast.
Seventh and most importantly, work with a salesman and a company with which you feel comfortable. You should work with someone who is genuinely interested in helping you realize what you have pictured in your mind. They should be experienced in dealing with weddings and not just with high school proms. They should be attentive to your wishes and knowledgeable enough to guide you to an outfit, which is elegant, befitting the occasion, and makes the groom feel like a million bucks!
In conclusion, the formalwear component of your wedding plans should be the least stressful decisions you have to make. If you pay attention to some of the issues I have suggested, find someone with whom you are comfortable, communicate your requirements clearly, and you will have a beautiful wedding with a minimum of hassle.