Put on your bra by placing your arms through the straps and leaning forward so that your breasts fall naturally into the cups. Reach around, and clasp your bra closed before standing up and adjusting the straps so that they aren't loose or pinching into your skin (ouch!). Make sure your nipples are in the right place by reaching inside the cup and giving your breasts an adjustment so that they're front and center. Sure this seems like a lot of work, but putting your bra on properly gives you the most accurate view of how it looks and fits.
Stand in front of a mirror that gives you a good view of your bra. Look at your breasts from the front and the sides to see if there are any big gaps between the fabric and your skin or wrinkles in the fabric of the cups. Bend over again to see if you easily fall out of the cups. Your bra should hug your curves if it's well fitting, so if it's the correct size, falling out won't happen, and those wrinkles and gaps shouldn't exist.
Turn sideways so that you can get a pretty good look at the band in the back (or turn completely, and peek over your shoulder). If it's taking a hike up your back, it is yet another sign that the bra (the band in this case) is too large for your body and isn't doing a thing when it comes to giving you the support you need.
Check out the straps on either side, and move your arms around a bit. If they're doing that annoying slide-down-your-arms thing, or if you already have a bra with straps that constantly slip, the cup sizes on those bras are too big, and you'll want to consider a smaller size.
Slip a finger beneath the band of the bra. One should fit comfortably, but if you can fit two fingers or more with no trouble, it's too large.
Ladies' Home Journal: Buying a Bra
"The Lingerie Handbook: Transform Your Body Transform Yourself"; Rebecca Apsan, Sarah Stark, Jill Wachter; 2006
"Fashion for Dummies"; Jill Martin, Pierre A. Lehu; 2009