Men
How to find your size:
As a general rule, a man's measurement is the same as his size. Where small, medium, large, etc. are concerned, the best way to ensure proper fit is to know your measurements. Always consult online merchant's size charts and fit tips.

What you'll need
A cloth tape measure and a full-length mirror. For best results take body measurements undressed or in lightweight clothing. Keep your arms at your side and have a friend take the measurements. When measuring, keep the tape measure snug, but not tight. Stand up straight!

Neck size
Measure around the base of the neck.
Easier method: Take a shirt that fits great and measure from the center of the button to the far end of the button hole. Inches = Size (If it's 17 1/2 inches, you're a 17 1/2 neck) Round up. For example, if you measure 17 1/4, round up to 17 1/2.

Sleeve measure
Bend your elbow and put your hand on your hip. Have someone measure from middle of back of your neck, around shoulder and elbow to wrist bone.
Easier method: Take a shirt that fits and fold at the elbow 90 degrees and measure from middle back of collar to cuff.

Chest size
Take measurements under armpits, around fullest part of chest and shoulder blades

Waist size
Measure around where you normally wear your pants, keeping the tape measure a bit loose or putting one finger between your body and the tape measure.

Hips
Stand with legs about six inches apart and measure around the fullest part of the hips.

Inseam size
You'll need a friend to measure from your crotch to where you want your pants to end, breaking at the top of the shoe in the front and just above the heel in the back (you'll need to be wearing shoes for this one)
Easier method: Find a pair of good fitting pants, and measure from crotch seam to bottom of pants.

Belt
Wrap tape measure over a good fitting pair of pants.
Easier method: Order the belt one size up from your waist size.

Hats
Measure above the ears about 1/2" around the forehead.

Gloves
With hand partially closed, measure over the knuckles, around the hand, excluding the thumb.

Tips

-If it's cotton, it's going to shrink. Add from 1/3" to 3/4" to compensate for it being washed. Some makers may have already accounted for shrinkage in the sizing. Ask customer service for a recommendation.
-Some manufacturers or stores may have their own special fit and size tips. It's a good idea to consult any additional information.
-Get a great tailor, because no matter how closely you order your size, it's still not custom made.
-Euro sizes are 10 higher than U.S., in jackets and suits. So a 56 Euro, is a 46 U.S.
-Cool tool to convert inches to centimeters - Find your measurements around the world
-Short men's sizes are usually sized to fit 5'7" and under; regular 5'8" to 5'11"; Tall over 6'
-Tapered fit or athletic fit in shirts means full shoulders and trim body.