One trick is to wear a fine knit sweater layered over a tight t-shirt without ever having the need to remove the sweater. A v-necked sweater over a crew-neck t-shirt in contrasting colors can look good.
Buy a longer fitting tee so that you get to see the bottom of it under the bottom of the jumper, this layered look will give the illusion of height thereby making you appear taller and slimmer. There should be a little "give" in the sweater so that instead of hugging your belly, it falls in soft creases around your middle.
Another trick which works well in the summer is to wear a loose fitting summer shirt - linen is good - left un-tucked over a pair of casual trousers.
Again, don't buy a size three times bigger than the size you actually need, as this will make you look bulky. Buy a size that leaves a bit of room so the shirt softly falls over your belly, and, if it's linen, again the material will create natural creases that will help disguise your girth.
:-)