1. slim weapons with short grips like Kahr P9, or officer's sized 1911s.
2. Extreme forward cant; get a 20 - 22 degree cant and that way the but will be pointing more straight up, even for J frame. On us thin guys, the problem with "FBI" cant or less is the the butt pokes out more to the rear and that's the point that prints. With a gun like an MK9 plus a 20 degree rake, the butt points more up. In fact, the MK9 with an IWB from HBE leather worn at 3:00 will be very easily concealed on you. That'll give you 7 rounds of 9mm in a package that is comfortable to shoot a LOT of at the range due to the weight. It's DAO and very very reliable. A J Frame would work in this rig as well.
3. Go IWB and avoid OWB. Since you're so thin you have no buldges to push your shirt out so it's a straight line vertically with the gun being a bump on the side. An OWB doesn't typically work good on that body style unless your shirt is heavier material and quite large. With IWB the pants/belt holds the shirt out and the gun closer to your side. If need be, spend the money and get pants 2" bigger and IWB is more comfortable. I wear 34's and in jeans I can IWB carry comfortably. In dockers or dress pants I buy 36's for carry and 34 for regular wear.
4. Finish your IWB rig with a decent 1.5" stiff belt and you'll be in pretty good shape. Avoid any holster that has anything more than a single layer of leather between the gun and the belt. Don't get holsters that have a clip or loop right on top of the gun. Be careful with any holster that has a reinforced lip or opening as this extra thickness can make concealment more difficult if it ends up between the gun and belt.
5. The further you go around the back from 3:00 the worse the printing will be from the butt of the gun until you start to approach SOB (small of back) and then the gun will be flat to your back and it'll conceal better but that's a dangerous place to carry a firearm.
Finally, you'll want to consider pocket carry with a J frame, Ruger LCP, Keltec P3AT, or the seecamps and NAA's. Personally I think the Seecamps and NAA's are either too costly or too heavy compared to the P3AT or LCP. J frames can be a bit bulky for some folks tastes due to the cylinder buldge.
Hope this helps
God Bless
Gideon