For precise readings, once a month at the same time of the day, measure your blood pressure three times in a row and take the average of those numbers. (Shoot for the time when you're most relaxed.) Fluctuations in BP throughout the day are normal, and consistency will help ensure accuracy.
Don't drink coffee before taking a reading. Studies show that caffeine can temporarily boost your numbers by 3 to 15 points.
Wait 30 minutes after exercising to give your BP a chance to stabilize. During vigorous exercise, systolic blood pressure can shoot up as high as 220 mm Hg.