For upper body, I often shop at H&M for formal wear, they have good slim cuts that fit nicely around my narrower shoulders and thin arms.
Tshirts are difficult, but I find thick, cotton shirts are best for hiding curves once in a while.
I tend to wear low rise bootleg or slim cut jeans (I had thin legs pre-T). Low rise as in it sits on your hips, not above. For me, I just find the smallest ones that I can fit my hips into, they tighten up the wide hips a bit and help disguise the butt. Make sure the legs aren't too fitting, or the shape might give you away. I like to shop at Bluenotes for my jeans, khakis and casual shirts, since they're cheap and have sizes down to XS.
However, dress pants for me have proven practically impossible to look completely masculine in, so beware.