Sorry Lixy, I have to disagree with you there......
I think there is a way to make sure your breasts don't sag (and I'm a 26 year old 38F, so I should know!)
Firstly, I'd say that a lot of breast is fat, so if you'd like to reduce your over all size, try losing a little bit of weight. You're evidently small in the rib cage (and probably every where else too) so if you do try to lose some weight, there's only one place it can go from - your breasts! Unfortunately, if you do lose weight in this area, it's going to make your breasts more saggy in the short term, as there'll be less fat being held in the same amount of skin, which leads me on to my main point:
To prevent sagging, make sure you ALWAYS wear a bra - the better quality you can afford the better, and if you haven't been fitted for one recently, then go and make sure you're wearing the right size - there's some scary statistic about the number of women wearing the wrong sized bra in the uk, but I can't remember what it is. I can highly recommend Fantasie or Rigby & Peller....they don't come cheap but look out for them in the sale in department stores or try looking here:
www.bravissimo.co.uk
Also, doing pectoral exercises to strengthen the muscles under your breasts will help to support them. Remember the old 'I must, I must, I must improve my bust' excercises?! That wasn't just a myth! Oh and good posture will prevent you from having back pain due to your bust, as well as improving the appearance of sag. Don't be shy, standing with your shoulders back and down will make your bust more noticeable, but only in the good way!
Lastly, I'd say be realistic in your expectations: the chances are, that you are the only person who thinks your breasts are sagging - probably because you're comparing yourself to your B cup friends. Large breasts are not going to be as perky as smaller ones, but no-one expects them to be. Doesn't mean they're saggy just means that they're real, and that's by no means a bad thing when it comes to the guys!!!
Oh and remember, that surgery can result in scars, which in the long term you might decide is more unsightly than a little bit of natural sag. That's what's always stopped me from going for a reduction.
Good luck my big breasted British sister!
Lou
x