If you want a proper fitting jacket or suit, the kind of snug but comfortable fit found on custom pieces, then you have to learn to speak your tailor's language. Ask anyone in the business—new school guys like Kirk Miller of Miller's Oath to the legendary Martin Greenfield—and they all agree. You don't need to master tailor terminology, but you should communicate honestly about how you want your suit or jacket to fit. No detail is too small. After all, their whole business is about small changes that have a big impact. Here's what to ask for the next time the tape measure comes out.
Your jacket's lapels should lay flat and touch the points of your shirt collar. The jacket's shoulders should hug your body's shoulders. Ideally, this fit should be mastered at the store, but you can have the shoulders brought in slightly via the back seam.
Off-the-rack suits are notoriously boxy in the middle. Have the sides taken in. The nipped waist will add shape and definition, broadening your shoulders and lengthening your torso.
You can always have the pants taken in or let out if you need more or less room in the waist, hips or seat. You can have them shortened, lengthened and (if there's enough fabric) hemmed with a cuff. Ask for very little to no break, so that your pants just graze the top of your shoes. Often, suit pants tend to fit a bit fuller, so you can have the legs narrowed or tapered to slim them down.
Sleeves can be shortened or lengthened, so they stop just above the joint of your wrist, flashing a quarter-inch of shirt cuff. And if you want your suit to fit like the ones seen in magazines and catalogs, you'll want to have the sleeves taken in or slightly tapered to fit your arms.