Im pretty short for my weight, so when I fight, most people tend to try to keep me at the end of their punches, with amatuer style combinations.
Heres what I do to get inside.
1.) FIGHT TALL! Yes you need to get inside, but fight standing up all the way, with good posture, because your opponent punches where you are at, not where you are about to be, so when you fight tall, and you decide to go on the inside, their is suddenly a dramatic difference in levels. Most guys who try to fight on the inside stay hunched over, and they make it harder on themselves.
2.) DONT WALK FORWARD! Instead, take ONE giant leap, and lean your body to either side as you lower yourself.
3.) BE PHYSICAL! Once on the inside, unleash, to the body double up your hooks, body head, push hold smother. If you corner your opponent, likely they will flurry with the intention to back you off, instead of stepping back, again, lower yourself, step to oneside and unleash to the body again, most of the time, they are coming forward at this time to get out of the corner, and they walk into the body shot.
4.) When you are on the inside, and not punching, your elbows should be in tight, and knees bent, so if your opponent throws a left hook to the body, you lower yourself, check it with the right elbow thats tucked into your side, and fire a right uppercut. And vica verca.
And finally, if you are on the inside, and plan to make it a short visit, ALWAYS end your combination with a stiff jab as you back away so they are stopped in their tracks momentarily.
Thats all I can think of.