While Im not currently training for any race in particular (though hope to find one soon!), keeping speed workouts in the weekly mix is very important. Its also a fun way of adding variety to your normal routine.
Typically, speed workouts are midweek, since long runs are usually on the weekends, but I'll post about them today anyway :) This workout that I've done is mainily for increasing 5k speed. A usual warm up around the track consists of me running the "wrong" way along the very outside or the far lane for two laps (1/2 mile) and then some pick ups (gradualy increasing the speed and length of stride over 100m) and dynamic movements (high knees, C-skips, bounding, etc). I'll then run another 2 laps at a medium effort to get my body ready for the speed work to come.
Then onto the workout: a series of 400m runs faster than 5k race pace with 200m slow jog recovery. Mine for example, if my ideal 5k pace was 8 min/miles, I am running the 400's with a goal of 1 min 50 seconds. I'll do between 4-8 of these depending on how much time I have, the temperature/humidity, how I'm feeling, etc.
After the speedwork, I'll then jog another mile or so, making each lap a little slower to cool down while contemplating my sanity.