As a general rule, keep the measuring tape firm, but not tight when taking your measurements. If you're measuring yourself, try and take them more than once, to average out any changes in your posture.
Guide to Taking Various Measurements:
Chest (men) - Simply measure around the fullest part of your chest, keeping the tape level under your arms and across your back.
Bust - Measure around the fullest part of your breast.
Waist - Measure around the narrowest part of your body between your bust and hips.
Hips - Another simple one. Simply measure around the fullest part of your hips.