good chin, usually have to eat a couple of punches before you get inside no matter how good your defense is. You should throw a larger volume of punches. A good jab to close the distance. power and leverage are very important too. Usually infighters have short arms so the make up for their lack of reach with torque and sitting down on their punches. Hooks and uppercuts will be their primary weapons thrown in close so those have to be polished. They should know how to cut off the ring and bob and weave effectively
Well, some people think inside fighting is all about tucking their head down and trying to get lucky with shots, and eating some hard shots in return. The ability to see what your opponent is doing in order to try and slip punches, roll with them, or tense up your body to lessen impact is a skill in itself. You can both be dishing out hard shots, but someone more experienced can take a lot less punishment. So yeah, the ability to see or read your opponent from close quarters is a big one for me. I think I learned this the hard way - by fighting bigger, stronger and more experienced guys. Those punches hurt - if you see them coming, you can avoid a lot of pain.
Be loose and comfortable in close. Be able to punch short and effectively; this means punching with your feet and hips, not your arms. Getting the right position- your shoulder into his chest, not squared up, which makes you a sucker for uppercuts.