Sometimes people want to hide their boobs. Doesn't matter if you're a trans guy trying to pass, a trans girl boymoding or a cis guy who's self-conscious about his breasts.
Since I have been doing this for a couple of years, here is the collection of strategies that I know.
1- Binder. This one is kind of obvious. But sometimes you can't get a binder on short notice, or you can't wear one for whatever reason. Or you can't or don't want to wear one that is tight enough to squish everything down to nothing. So, the other ones are for you.
2- Layer technique. This works well for small breasts and for breasts that are more wide than perky. Wear as many layers as you can stand, going from tight-fitting at the innermost layer to loose-fitting at the outermost layer.
3- Choose the correct top layer. This also applies if you only wear a single layer. 3 factors play a role in the fabric: Color, stiffness, and fit.
When it comes to color, black is the way to go. As black as you can find. If you don't have any black, the darker the better. Most of the time, shadows are what gives away the shape. Uniform color is a must, avoid patterns or prints, as those also give away the shape.
A stiff fabric will hide the shape underneath. Textured, thick, stiff fabrics work the best.
Fit is important. I find that going for about 1 size too big gives the best results, it just looks like a comfortable fit. Anything that actually fits will hug your body shape too much, and anything larger than that just looks ridiculous (sorry, every dsyphoric trans guy ever, it's the truth).
4- Don't sweat it. If people want to see you as a certain gender, they will see you as that gender and ignore the parts that don't really fit. It is not unusual even for cis men to grow boobs. People who don't engage with the trans community much won't recognise the signs, and those who do are pretty likely to simply accept whatever you're doing.