Respect the tape measure.
A truly custom suit should reflect who you want to be, but it has to flatter the figure you actually have. Now is not the time to be in denial about your height, your waist, or any other lingering insecurities. The tape measure doesn't lie, and the measuring process is only awkward for men who don't want to face themselves. A good tailor can compensate for a long torso, a short neck, arms of different lengths, and features far stranger than anything you're likely working with. Just make sure he knows how you feel about the suits you already own. Are they uncomfortable through the shoulders? Do trousers sit too low?
A tailor will measure everything from your inseam to your arms and some will take photos of you to capture your posture. Some have been known to ask what side you prefers to wear your manhood. Just answer the questions.
And please have mercy and do not suck in when your tailor is measuring your waist. You're not fooling anyone, and you'll end up paying the price when you have to go on a liquid diet to fit into your new purchase.