When it's time to buy a suit, you should go in with an idea of what you're looking for. Color, fabric, and number of buttons are just a few examples of the choices you'll be making. No matter how much you're looking to spend (see previous post Looking Good on a Budget) you can find something that will look great if you know how a suit should fit. If you're buying off the rack from a department store, it's especially important to know your stuff. Here are some tips; some ideas and pictures are from GQ:
1. Don't trust the sales guy. First of all, take a look at what he's wearing and how he wears it. It's a pretty safe bet that it's not going to be what you want, so you've got to be wary of what he tells you. Most of them don't know much about how a suit should fit, or at least how you want yours to fit.
2. Know what fit you like. Specifically, do you like a snug fit or a looser fit? Sounds basic, but if you know what fit you want, and you see a suit that doesn't fit your mold, you can move on immediately.
3. Know your size beforehand. Lots of times the sales guy won't even measure you. They'll just say "oh you look like a..." and then have you try on a jacket. I recently went into a department store in preparation for this post to see what the procedure was, and I wasn't impressed. The guy would have had me leaving with a suit that was at least 1 size too big. Once when I was younger and naive about suit buying, the sales guy got me into a suit with a pair of trousers that were a size 35 (I wear a 31 or 32). I have no idea why, but he had me convinced that alterations could fix any issue. NOT TRUE. Alterations are more for fine tuning. My size 35 pants, which are doubly bad because they're pleated, make me look like I've got balloons in my pockets.
Now for the various components of a suit that you should be familiar with:
4. Shoulders: The suit's shoulder's should hug yours. The shoulder pads shouldn't stick out past your shoulders. If you stand against a wall and the suit touches the wall before your arm does, the suit is too big.
5. Chest: You should allow for a fist's worth of space between your chest and the button when you have the suit buttoned. Not too tight though--you shouldn't have to strain to button it.
6. Length: With your arms hanging down, you should be able to cup your fingers under the sides of the jacket. Some styles have the jacket fitting shorter than that.
7. Number of buttons: Now we're getting more into the style of the suit. A 2 button suit is the classic, and is currently the most popular. The 3 button was popular for quite sometime, but can make you look '90s if you're not careful. If you can find one that doesn't look too high cut (you're not in the NBA) and that preferably has a roll-over lapel (you can button the top button or just do the middle button, the soft lapel will naturally roll over the top button) then the 3 button can be an acceptable choice. A 1 button suit is a bit more rakish but can be a good look if you're a bit more daring.
8. Vents in back: A center vent is all-purpose. Both modern and traditional. Side vents (2 vents on the side instead of 1 in the center) is a bit more stylish. No vent is a no-no.
9. Lapel: A notch lapel is the most common. Always a safe bet. A peak lapel is what you usually see on a double-breasted suit, it's the lapel that points upwards instead of sideways. They are quite nice on a normal suit as well, they look a bit more elegant.
10. When you're trying suits on, make sure you're wearing dress shoes, either your own or borrowed from the store.
11. The pants: they should fit comfortably, the rise shouldn't be too high or too low for your taste. Plain front (no pleats) is classic and also very popular right now, as is no cuffs, although cuffs are still very popular.
12. What alterations can and can't do: If the suit doesn't fit in the shoulders, move on. If a salesman tells you shoulder pads can be reduced or reshaped, they are lying. If pants are an inch or so too tight or too loose, that can be fixed. More than an inch (like my ridiculous size 35's) and it's a bad idea. Sleeves can be lengthened or shortened a bit. The sides can be tapered (and should be if your body calls for it) so it fits you more closely. If there is a roll in the fabric on the back up by the neck, alterations can usually fix this.