Matt's post provides a great link and James comments about interval training are equally valid.
When training with friends to help them pass their fitness tests we used a variety of exercises. Here's some of them
Sprint 5 metres, walk back, sprint 10 metres, walk back, sprint 20 metres, walk back, repeat 10 times, rest for 2 minutes and repeat the exercise again.
30s run, 30s walk, repeated 20 times, 5 minutes off, then repeat the whole exercise, 5 minutes off, then repeat the whole exercise - running should be at 75% max intensity
2mins run, 1 min walk, repeated 8 times, 5 minute rest and then repeat the whole set again.
Run 4 minutes, walk 2 minutes, run 3 minutes, walk 1.5 minutes, run 2 minutes, walk 1 minute, run 1 minute, walk 30secs, run 1 minute, walk one minute, run 2 minutes, walk 1.5 mins, run 3 minutes, walk 2 minutes, run 4 minutes. Rest for 5 minutes, repeat exercise, rest for 5 minutes, repeat exercise.
Run a touchline of a field, walk the other 3 sides, run touchline and goal line and walk other two sides, run touchline, goal line and then touchline, walk the final goalline, run a whole circuit. Take 3 minute rest, then complete the routine in reverse so you start with the full circuit run and work down, take 3 minute rest, then work through another routine back up to the full circuit - running should be at 75% max intensity
Jog from one corner post to the centre mark, turn to the right and sprint to the other corner on the same goal line as your start point. On arrival at the second corner, do 5 sit ups. Jog to the centre mark, turn right and then sprint to the corner opposite the corner you started front. On arrival in this third corner do 5 burpees or press-ups. Jog to the centre mark, turn right and sprint to the fourth corner, where you do 5 start jumps. Jog to the centre mark and sprint to your starting corner. Rest for 5 minutes and repeat. Rest for 5 minutes and repeat.
Last one for now ...
Start on the goal line and run to the edge of the penalty area and back to the goal line, run to the halfway line and back to the goal line, run to the opposite goal line and back to the goal line. Take a 1 minute rest and repeat. Do 3 sets of 8 (24 runs in total) with a 3 minute rest between sets - pace 50% of max intensity.
Most important things in training are the warmup and warmdown - see Matt's link for good practice.