I bought some white undershirts a size smaller to wear tucked in with another shirt over. Basically for a layer between the holster and my skin. Also bought a few casual button down shirts designed not to be tucked in.. more squared off along the bottom then rounded. Neither were really needed and using the holster I have chosen I can wear the sidearm all day even without a layer between it and my skin.
I wear a waist 36-38 pant and didn't need to upsize for an IWB holster.. MTAC by Comp-Tac.
Really.. The only thing I'd suggest is a high quality leather belt. I have a couple of the kydex reinforced contoured belts from Comp-Tac. The $99 price was hard at first.. it's a friggen belt! However, after a couple years of daily wear I can fully say it was the absolute best purchase and well worth the money. Supports the holster & firearm (Walther PPQ) with no issues, well made and I have no doubt that they will last a long long time. After 2+ years they show hardly any wear. No separation, no distorted shape, holes still perfectly round, etc.
When first starting to carry everything screams you have a gun! It's a mind set that I think many, if not most, new permit holders go through. I know I did. Now I'll toss on a loose fitting t-shirt over it and head out. I honestly don't give it a seconds thought anymore.