1. Go to any men's clothing store that sells suits: Jos. A. Bank, Men's Wearhouse, J. Press, Brooks Brothers, the suit section of any department store...
2. Act interested in buying something. A salesperson will approach you within 2.4 seconds. (They mostly work on commission.)
3. Tell them it's been a while since you've had your measurements taken--could they do your neck, sleeve, waist, and jacket size?
4. Write down the measurements. Thank your salesperson and indicate that you're going to see if you can find something that strikes your fancy. This should get him/her to quit hovering.
You can rely on the neck and sleeve measurements. Relying on the waist measurement is also probably safe, though they tend to give you a spare inch.
As for the jacket measurements: subtract one increment, then go find a jacket in that size (e.g. salesman says 42, find a 41 jacket). Notice how well it fits, drapes, and flatters your silhouette. Now, try a jacket in the size the salesman told you. Especially if you went to the Wearhouse or JAB, it should fit like a tent. Try the suits one increment in both directions just to see which fit you prefer. There is some room for taste in these things. And remember, if the shoulders are perfect but, say, the waist is baggy, some of that can be solved with cheap alterations. Getting shoulders altered, on the other hand, is expensive.
N.B. If you are at J. Press, the jacket that is an increment smaller will also fit like a tent. This is because J. Press sells what are called "sack suits", a staple of a style called "American Trad" or "Trad", which is complex enough a topic for another post some day.