Diamonds, I'll be the first to admit that I loathe the way my body looks! I look like a 12 year old school boy got in a fight with a carrier bag full of mashed potatos and lost, but for the sake of my norks I went and got measured and now I wear bras that fit perfectly and that I'm proud to be seen in!
All I did was go in to Marks and Spencers, whip my jumper off and they measured over my T-shirt and found out my actual size. Took a total of about 2 minutes. I must admit I did blush a bit but it was over so quickly and I found a gorgeous set in there that has now become my lucky underwear! 
It's recommended that you wear a non-padded bra (I don't actually own such things as 'non-padded' bras but i just wore the one that was the least... enhancing) so that they can measure your bust properly without your boobs being harnessed under your chin.
Also, like jeans, different shops have different sizes so it's always best to try it on before you buy. Whilst in one shop I'm a 32A in another I'm a 34C so it really is worth giving it a go!