AshleyFanjo, I think the reasoning is:
No high necklines because it maximises the distance between neckline and boobs - makes you look saggy.
Likewise too short tops or tops tucked in minimises the distance between boobs and bottom hem of garment - more saggy appearance.
A skinny rib poloneck, tucked in, is the ultimate in both of these, PLUS ribs which stretch and distort over your boobs, drawing more attention to them.
BUT ... you can get away with it, if you're tall, especially with a long torso, slim, reasonably pert and not THAT norktastic. I wore that sort of thing at 16, without the benefit of being tall, but reasonably long in the torso, young and pert, fairly slim and a 36DD. At 37, no longer any of the above, and 36G - no chance.
Vulgar - minimisers are rubbish, everything ends up under your armpits in a horrible unflattering shape. You can shop Bravissimo online, you just need the faintest inkling of a clue what size you are (be brutally honest about what you wear now). Order as much as you can possibly afford, and send back what doesn't fit. They process returns quickly so if you time it right you won't pay for what you return (ie order right at the start of a credit card period, if that's how you do your shopping online).
For online bra shopping it really helps to understand bra sizing, particularly the fact that cup sizes go up with band sizes, eg that the OP's 40DD is actually two cup sizes larger than Ashley's 36DD, and smaller than a 44DD. Really useful when you find that, say, a 36E fits great in the cup but is big in the band. You don't then get a 34E, you get a 34F.
Oh, and underbust seams ARE the work of the devil, except when they manage to actually sit under said bust (horrid word), in which case they are fab.