You have to dress around the weapon. Unfortunately, some weapons require more "dressing around" than others.
lwestatbus, I may have missed it, but I never caught what sort of pistol you are trying to conceal.
Thinner pistols are generally easier to conceal, although even some thin pistols might have some corners that are just "print prone."
I have found that dark colors conceal much much better than lighter colors. I can easily make my 1911 disappear under just a black or navy T-shirt, with an A-shirt (AKA wifebeater) as an undershirt. If I tried that with a lighter color shirt, the printing would be noticeable on the beavertail. I doubt that others would think "gun," but it's enough to make me look for darker colored shirts when I'm getting dressed to go out and about.
As mentioned by some others, patterns do help break up the print as well.
For polo style shirts, I know someone who is a LEO, and his department sometimes gets free stuff from big-name distributers like Blackhawk, 5.11, etc, so sometimes he gives me any free stuff he doesn't want. One of the things he gave me was a 5.11 polo with a velcro front pocket, in navy. It's probably one of my favorite warm weather concealment shirts. It does not say "5.11" anywhere, so it won't make you look like "Tactical Ted," and the velcro pocket is great for a cell phone. It won't fall out if you bend down, and it frees up your pants pockets a little more. It conceals my 1911 very well. I have no idea how much they run price-wise though, but I will probably pick up a few more eventually.
Jason