Get yourself measured. It's one of the biggest favours you can do yourself. You can try measuring yourself but it's almost impossible to do accurately.
I've found that Shock Absorber are pretty accurate with their sizing if you have a professionally-done measurement.
Sam
do get yourself measured but don't take it as gospel. I was told by an assistant in a dedicated bra shop (a proper one, not la senza or bravissimo, but one of those ones that sells no nonsense bras) the only accurate bit is the measurement around the ribcage (as it happens I actually am smack bang between a 34 and a 36, but unfortunately they don't do 35 bras, but it doesn't matter anyway. The woman told me you should only use it as a guide as all bra manufacturers are different, particularly in the cup size. A "C" cup in one bra can be the same as a "B" or a "D" in another.
Therefore, you should ALWAYS try a bra on to see how it fits.
When trying on sports bras i wouldn't recommend buying one of those that has a double layer of material (so they look like a top rather than a bra) as you can't really see if your boobs fit in the bra properly.
I would also recommend bouncing around like a maniac in front of the mirror to see how much your boobs move to make sure you have the right size (mine don't at all in my shock absorber) and also, don't be afraid to lean forward and jiggle your boobs in the "cycling position".
And yes, as said, keep an eye on how old your bra's are, as the fibres start to give after a time (i must admit i didn't know this until recently and had some REALLY old ones) but also, i think i may have already said this, but drying by a heat source (i.e. radiator or tumble dryer) is a no no.
I've seen women with big boobs jogging without a sports bra and it makes me wince!
With shock absorber, i think the bra's they recommend for cycling are there level 3, but to be honest, i ain't taking any chances, i go for the top level!
i did write to shock absorber and told them we need a "cycling only" bra because of the position we are in on a road bike. Whether they listened i'm still to find out.
if you order from Next and have them on account, you can order different sizes and then return the ones you don't want, without having to pay for them all up front. Then just pay for the one you want. Triumph have a good reputation too but i prefer shock absorber
. I wear my sports bras nearly all the time now. I think I got used to the comfort wearing them every day for months on tour last year and it felt weird going back to underwires. Luckily I picked up a not too sport looking one in the US and it's fine under my work clothes.
). The cups are the wrong shape for my breasts. The other problem with bigger cups sizes is that they are sometimes too round - I think they are either designed for younger girls who don't droop so much 
or more - um - enhanced ladies.
rather than the 'monoboob' sausage (so not good) that many sports bras create.
