jab jab jab........ till you get there. if your out of range for your jab dont bother throwing anything else. This is the simplest and most effective ways to learn to get inside on the taller and longer guys. Once you've got the jabbing in down, start adding level changes and feint, jab jab...... get this first, then move on to other techniques.
Note: your jab doesn't have to land on the chin, gut of the taller guy. Land that jab at the closest thing to you (gloves, arm) and keep jabbing until you get it to him, than jab the chin/gut, and put together your power shots in your range. Don't try closing the gap by launching in out of range with 1 big step into your jab. instead, cut that distance down to 2-3 smaller quicker steps and add a jab with each step trying to hit the closest thing to you and work your way in. Simple, basic, but nonetheless effective against both novices and experienced guys that are taller and longer.
This works in MT also, despite popular opinion (of some). Good thing to add in MT for the shorter reach/height fighter is to throw your leg kicks at the end of your combos as the taller guys trying to get out, then jab, jab, jab, repeat............ That low kick at the end of your combo as the taller guy retreats will cut down the time and distance you'll have to cover going back in.