What I have noticed with my 800 meter runners is by them doing sprints from 30-100 meters once a week has allowed them to stay fast and sharp throughout the year.
To me the key is allowing full recovery between sprints in order to keep it a sprint and not a workout. I have noticed that if you do this on your recover they seem to respond just fine. I almost like to keep the sprints 50-75 meters to asure they don't get sore or fatigued. The reason im worry about soreness and fatigue is because most of the time I do these short sprints on recover days.
Although I will do them after some tempo runs. For example: 15 min easy, 30 min tempo 10 min easy, then 3X 100 Build Up, 5 X 50 Sprint.
I like the suggestion one poster posted about 8-10X 100, this is a very good workout but it does become a workout and can effect the rest of the weeks training.
Im not saying I know all, just my experince going over 75 meters seems to become more fatigued. A pure sprint is really only for 50-60 meters anyway. After that such as 100-200 becomes more speed endurance. Which is an important part of ones training and shouldn't be neglected. I just think you can get away with both if your training schedule is set up correctly.
Sorry for poor typing and spelling skills