Always measure against the body including any undergarments (thermals, t-shirts etc) that would be worn under the outer garments.
Measuring your chest:
Measurements should be taken from under the arm at the fullest part around the chest horizontally and around the full girth of your body
Measuring your waist:
With your feet placed together, measurements should be taken from around the natural hip line (put one finger between the tape and your body before you measure)
If in doubt!
go up a size.