Walmart has wrangler jeans that are infused with spandex. They look just like jeans, but they stretch. They are extremely comfortable for IWB holsters ( and Thanksgiving dinner

)
And like Anni said, get your t-shirts a little larger than normal for the summer time, the rest of the time wear a button up shirt that's a little large and leave it untucked and unbuttoned.
And always, whether I am carrying IWB or OWB or not at all I wear a " Wife beater" t-shirt under my shirt. So yeah, technically 2 t-shirts. I tuck the wife beater in and then put the holster between the wife beater and my jeans so it isn't riding against my skin. Then I put either a baggy t-shirt or an unbuttoned short sleeve shirt on over that.
You won't even know it's there.