Posted: 07/08/2014 at 10:51
Stay away from the plum varieties - Roma, San Marzano, etc - to avoid thick skins. Unless you're intending to make sauce, in which case the thicker skins are an advantage.
Sounds from Buddyboy's experience that it's not variety-related. Never grown those varieties so I wouldn't know. In addition to his suggestions, overfertilising can also cause thicker skins.