There's a particular kind of top, usually anything with a bit of a neckline or something fitted through the body, that looks completely different depending on what's underneath it, and that's the moment I reach for a push up. A plain t-shirt on a Tuesday doesn't need one. A wrap dress, a going out top, anything that gapes without a bit of help, does.
These bras don't age the way a plain cotton one does, because the padding is doing the work and padding has a shelf life. Expect the foam to flatten a bit after a year of regular wear and washing, and the shape softens before the straps or the band actually give up. Hand wash if you can be bothered, and if you can't, a lingerie bag on a cool cycle stops the hooks snagging the padding and pulling it out of shape. Never tumble dry one, the heat does something strange and permanent to the foam and you end up with a slightly lopsided bra and no idea why.
What it doesn't get on with is anything you want to look natural or sporty in. Fine knits that cling will show the edge of the pad, and anything backless or very low cut is asking for trouble because most push ups have a fair bit going on at the back and sides. Save it for structured fabrics and proper necklines, and keep a separate plain bra for gym days or soft jersey, because the two jobs don't overlap.
In the shop, check the gore, that's the bit between the cups, actually sits flat against your breastbone rather than standing off it, because if it's floating you've got the wrong shape for your chest, not the wrong size necessarily. Check the padding is angled rather than just a flat shelf of foam, angled ones give lift without making you look like you're wearing armour. And have a feel at the underwire ends, if they're poking or scratchy in the shop they will be a nightmare three wears in.
Fit is where most people go wrong with these specifically, because a push up that's too big in the cup just collapses the padding inward and does nothing, and one that's too small spills over the top edge, which is its own kind of alarming. Go up a cup size from your usual if you're between sizes, rather than down a band. Check the back isn't riding up above the front line when you lift your arms, that's your real sign the band's too loose, whatever the tag says.
Tomorrow I'd put mine under a fitted shirt with the top two buttons undone and a decent pair of trousers, nothing clever, just a bra that's actually doing its job under something that shows it.