Hiding one's breasts is a challenge during warm summer days. How much of a challenge will depend on your unique situation.
My breasts aren't large -- probably a good A cup -- but my nipples constantly stick out and are very noticeable beneath a T-shirt. I bought an Under Armour compression shirt. It helps push my nipples in a bit, but they're still very noticeable, and wearing a compression shirt is not what I would call comfortable. A sports bra is no better at compressing the nipples. I haven't tried pasties.
If you're overweight, getting away with large breasts is easier because your belly protrudes enough to hide the boobs. It's the long pole of the tent, so to speak. Gynocomastia on overweight men is common, so nobody thinks twice about it. I'm not overweight, so my boobs have nowhere to hide. In a somewhat tight T-shirt my chest looks like that of a developing teenage girl, and the more I slim down and lose muscle in my upper body, the more feminine I appear. No matter what people attribute the growth to, it looks odd on a gray haired 60 something. Some people are more observant than others, so there are times I'll get a fleeting look by a stranger if I'm dressed in a T.
My solution is first to condition myself not to care what people think. Right now I'm rather self conscious about my boobs showing. I expect that in time I'll get used to it, and forget about it. No one is going to use it against me anyway, so why fret?
Second, I don't plan to grow my breasts to mature woman size. I've already cut down my PM to a low dose -- enough to keep the mental benefits -- and my breast growth has slowed in response. I like small breasts, anyway, so it's not a problem for me to have a goal for small breasts myself. That has always been my intent. It makes the challenge of hiding a lot easier. I plan to use breast forms to augment my figure when Clara wants to look "oh so feminine".
I know it's getting off the topic, but it needs to be pointed out that growing breasts is going to change your life in many ways. Once you have them, they're here to stay. Make sure you understand that before undertaking your NBE quest. Set a reasonable, practical goal so that when you reach it, you are prepared for the consequences.
Clara