This depends on a lot of things.
1) I am so flat, that push up bras or enhancers just don't work for me - there's nothing to push up
2) I'm also so flat I've given up on wearing bras that fit - I don't need the support so as long as it's comfortable and padded I don't see the sense on paying extra for a bra that fits so I just remember not to bend down unless I keep a straight back as my bras are prone to gaping and showing off my boobs.
3) I really think if you're very small, padding and well structured bras work well so I wear bras with really rigid cups and that are quite padded - they are prone to the gaping, but they also structured enough that you can't tell they don't fit, they just look like my boobs under my clothes (people don't believe how flat chested I am because I fake it so well
4) The right size varies from shop to shop and from style to style so you just need to try on loads of sizes to find the right one for each individual bra and work out what shapes suit you - it takes a lot of time, but once you get a general idea it gets easier
I don't really know how much 1 and 2 will apply to you, for many women, push up bras work well even if they're tiny, I'm just beyond tiny so they don't work for me :)
I hope you find what you need
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