As condition1 said, the smaller the "easier" as far as concealment... If it's REALLY a concern, you could even look at some of the pocket pistol offerings (i.e. the LCP, etc)
With that said, how do you know you are "printing"? Has anyone mentioned it? Or is it just because you see/feel it yourself? We tend to be very critical of ourselves. Standing in front of a mirror moving around you think, "Oh geez! I can see the gun clearly!"... But in reality it's just because it's you and you know it's a gun... Others who saw the same thing most likely wouldn't even notice, and even if they did notice they'd think cell phone, ipod, or any number of other crap that people carry around on their belts.
If it really IS a problem, try changing the supertuck around. Different positions are not only more comfortable, but also often print less... Condition1 said he prefers the 2 o'clock position... I've found for me the 4 o'clock position offers the most comfortable ride while still eliminating printing.
Also, don't just MOVE the supertuck... Adjust it as well... You can adjust the cant as well as the ride height... You may find that having it ride higher may actually print less despite what you would normally think... changing the cant can also affect comfort and printing...
Make sure you have a GOOD belt... I know it's stressed all the time, but it really can't be stressed enough just how much of a difference a GOOD belt can make. A good heavy gun style belt will not only keep it locked in place, but it can also pull it inwards slightly eliminating print.
Last but not least, going back to ride height, you can also try going deeper... If you have a supertuck with the stock clips, try getting the velco hooks instead... Get a velcro backed belt (or do it yourself) and now you can plunge it down in your waist band much further...
That's about all I can think of short of the type of t-shirt... All t-shirts aren't created equal... Stick with ones that are heavier and thicker as well as looser/larger and obviously you'll print less.
