As others have said, tailoring is your friend. The other option, of course, is figuring out what brand of shirts works well for you. After I found Nike Dri-FIT and Delta shirts, that's most of what I get. Sometimes there are variances, but mostly I can buy what I've determined is my size on the internet and it fits.
You can also check out a discount retailer such as Nordstrom Rack or Burlington Coat Factory. Just go in and try stuff on until you find what you're looking for. If you don't find anything, you can come back in a few weeks and there will be a bunch of new/different items. The likelihood that you won't be able to find something good after a few trips is low. Wear your gear like on any other normal day, and you'll see how it conceals right there in the mirror.
All of this applies equally whether we're talking t-shirts, polos, or button ups.
This is a good rule of thumb (or wrist, as it were).
For me, I like when t-shirts hang right around/above where the hot dog attaches to the pubic bone. Sorry to get graphic, boys and girls. When doing very aggressive draws, I can practically slap myself in that region while trying to get a hold of my shirt, and nothing detrimental happens. When I have shirts that are a little longer than that, I have sometime tapped myself in the balls. Gets old very quickly.
After seeing what LSP972 wrote, I stood upright and checked out where my wrist falls. Guess what, right in the same place I was talking about liking my t-shirts, on my body anyway.
This is certainly an option. The other option is to get a snap button sewn in underneath the last button. So it looks like shirt is buttoned normally, but the tails unsnap easy peezy.