Its more fun just to eat more pizza, but if that isn't for you go for the smart carry or a belly band.
As far as the 1 size larger shirts, it will depend greatly on what the shirt is. I have many Perry Ellis shirts that I already have to get in XXL, any bigger and they will just look out of place; so I carry at approx 2:30 with these on. Normal "polo" type shirts you can get away with 1 size larger, and add some prints to boot to help break up the printing.
Shop at a CCW safe place so you can take clothes in to the dressing room and try them all on and do all the twisting and squatting/stooping you need to so you feel more comfortable. This will also teach you to not touch or tug at your shirts while CCW, which can tell those who do pay attention that you are uncomfortable about something.