wind sprints are your best bet. I do super fast (but short) bursts. For example, i'll jog, then I sprint for about 6 seconds (about the time it would take fora 5 punch/kick/knee combo for example)...
In sparring, you'll find that you'll need short bursts of explosiveness in your combinations, then there might be a lull in the action. Try doing 20-30 short bursts during your run.
Another drill: at the end of each 3 minute round (the last :30,) do 15/15. 15 kicks on each side in rapid fire, power kicks.
Another drill: one straight minute of left/right on the bag
another drill: shins: you and your partner stand facing each other; the first guy throws a low power (30%) kick to your lower shin outside. You do the same back to him. Then he throws one a bit higher, then you match it. Then he throws a kick to the upper thigh, then you repeat. Then you go to the inside of the legs. Then you move up to the ribs, then the shoulder, then the head. BUT, with the head kick, the reciever will tuck his arm in very tight and use his fist up close his head to absorb the kick.
This drill will not only toughen your body up, but will get you used to getting hit. Its a great drill!