Inside-the-waistband holsters make even full sized pistols seem to disappear under little more than a T-shirt, and I rely on them a great deal in the summer.
A good holster must be made for your pistol. Generic, one-size-fits-many models don't do a good job of retaining your pistol and are generally not very comfortable.
A good holster should be molded tight enough to retain your pistol without straps or tension screws, and it should cover the trigger guard. It should also have a reinforced mouth that keeps the empty holster open for easy one-handed re-holstering. The holster's belt loops or slots must match your belt so the holster stays put for all-day comfort and a sure, speedy draw.