Warning: This post uses the words "breasts" and "bra", two very cringe-worthy words, quite a bit. You'll get used to it.
Before I show the tips, just note, that this will NOT give you the ever so popular G-cup sized breasts you see on characters like Rangiku, Orihime, Tia, or... 90% of the female characters from Bleach. In order to do that, you would need to be close to that size already (why would you need these tips?), or get plastic surgery (which is a little silly for cosplay unless you love it that much). The changes are noticeable, but it's not in a "magically grow 3 cup sizes" way. The tips will help your breast appear fuller, and create the illusion of cleavage, but that's it. They're also a little uncomfortable, so don't use these everyday.
Stand up straighter: The simplest tip of all. If you aren't completely tsurupettan, you can make your breasts appear larger by standing up straight. This would only work under clothes, but this should be used in conjunction with the following tips. You shouldn't slouch in photos if you want to look your best, by the way.
Wear a smaller bra: I do NOT recommend this. However, it's here for reference. This is usually one of the things people try, but it's a bit like wearing the wrong shoe size. It hurts and is probably bad for your health. It leaves unsightly red marks or bruises, can compress your ribs and make it hard to breathe, cause back problems (not sure about that) and damage the tissues in the surrounding area. Trust me, constant rib pain is not fun at all, so don't do it to your body on purpose.
Wear two bras: It actually works, but is a bit uncomfortable. It gives a slight appearance of cleavage (you would probably have better results if you weren't flat like me), and makes them look larger overall.
Push up Bras: Push up bras are special bras with padding in the bottom to "push up" your breasts and make them look bigger. They're great for creating the illusion of cleavage or larger breasts. Some have different "grades" of effect. However, some of them can be a little expensive.
Padded Bras: Padded bras have padding inside to simply make the breasts look larger. They don't "push" anything.
Bra inserts: Honestly, I don't know what they're called. They're usually made of cotton or silicone. You put them inside the bra, and to the lower side of, or underneath the breasts. Many people just use socks... but I think that sock sound painful.
Makeup: For added effect, you can apply makeup to your newly created cleavage. Apply a bronzer or darker shade of foundation to add "shadow" along the sides of your cleavage, and a very subtle shimmery powder to the visible parts to add a highlight. Don't go overboard. It should look relatively natural.
Fake it: Yes, this entire post is about faking larger breasts and cleavage, but in this case I mean, just make some. I linked to a tutorial on how to make fake breasts in my post about crossplay. Usually, you can find good tutorials for crossplay or crossdressing. It sounds weird, but women can learn a lot from them.