Step off/long hook.
At the gym I train at we train in multiple martial arts (I hesitate to call it mma because, although we incorporate what we learn in each art into the others, we actually train in separate arts as apposed to "mixing" them all together) and we have a very strong emphasis on sport kickboxing. One of the strategies that comes from this incorporation is what we call the step off/long hook.
Say you are squared up with your opponent while sparring, well most the time the first thing you want to do is throw a jab, right (as this is your set up to most combos/techniques)? Well as your putting your jabs in, after your have your opponent "conditioned" to what he thinks you are doing, the moment you throw one, step off at about a 45 degree angle to the outside with your lead foot. When your jab arm is fully extended, you do what is called a tai-sabaki with your back leg, which is actually a judo body turning technique where you bring your back leg around in a swooping motion (guess you could call it a "moon step") around, ending up outside your opponent perpendicular to them. Now, as you threw the jab that put this in place, you are not going to immediately retract it as you would a normal jab, but instead, when your arm is fully extended, you turn your wrist in, turning your jab into a hook. It may sound like the hook would be pretty powerless as it starts from an extended jab, but if you do the tai-sabaki right, the weight and momentum behind your body turning actually puts a fair amount of force behind it. Now, once you're perpendicular to your opponent and have landed the turning hook, you force them to turn into your "hallway". The second they do this and turn to face you is a perfect opportunity to land in a powerful straight off your back hand and follow it up with whatever combo/technique you feel from there.
I hope this helps, sorry if its kinda confusing, i've never tried to explain a multi step technique through writing before

, but this is one of my favorite set ups and i've probably landed more big straights off this than any other.