When it comes to single or double-breasted, it's largely a matter of taste. Double-breasted suits are generally more formal.
Your sleeves should finish right on the wrist and your shirt sleeves should be a quarter-inch longer than your jacket.
If you are slim, you may be able to get away with a jacket with one button but, personally, I prefer two-button jackets.
Whether you go with the trend and buy pants that are slim-cut or more traditional, it's best to avoid any that are too high-rise or low-rise.
The suit's shoulders should hug yours and the shoulder pads should not extend beyond your own shoulders. If the shoulders are too big, there will always be too much material to be taken in.
You can get trousers taken up or in, but try to buy as close to the correct size as possible. It sounds obvious, but you'd be surprised by your decisions when faced with a heavy discount.
You should be able to button the jacket easily without it straining. There also shouldn't be too much space between the button and your chest.
Those flaps on the back of the suit are called vents - one in the middle is traditional but other styles have them off to the side.
As a rule, the wider you are, the more vents you can get away with.
