11. Increase your base miles. Many people try to race and set new PR's after not completing an adequate macro cycle. In other words, they don't spend enough weeks going through the necessary training phases before goal races. If you are striving for a PR in a particular race, you need to carefully prepare for months in advance. Of course, the length of the preparation period/macro cycle depends on the distance of the race you're preparing for. Even if your goal race is a 5k, you will have a much better performance if you started your training/preparation for that race by building a base of at least 25 miles per week. If you are training for a goal 5k and don't plan on racing any longer distances, I still recommend building your once weekly long run to at least 9 miles.This will make it possible for you to taper effectively, which could will be a huge factor in increasing speed. If you have not done this in the past, you may find that it is the secret to greatly increase your speed and have a breakthrough race.
12. Eat more protein. Of course, your whole diet is important, but I have particularly found that many athletes do not get sufficient protein for the quantity of training they do. I suggest keeping track of your protein intake for a few days. It may be that your muscles cannot get stronger and can't recover properly because you are not consuming enough protein. I recommend listening to some podcasts about nutrition for athletes. A highly recognized national authority on nutrition for athletes is Nancy Clark, MS, RD, CSSD http://www.nancyclarkrd.com/index.html. It could be that all you need to achieve a better time is a better diet.
13. Take a long, hard look at your running schedule. Does it make sense? It is carefully planned or haphazard? Are you training to be fit or is your training specific to what you need for the events you normally compete in?
14. Avoid any intense training sessions the week of any goal race. By that I mean a race in which you hope to set a PR. The only hard training you should do during that week is race specific. Even marathoners should not be doing any high mileage the week of a race. This will put you in much better position to run fast. No weight training, no boot camp, no spinning.
15. Take a weekly yoga class. This is an excellent way to improve core strength, increase balance, and work on flexibility. All will make you a better runner, and improve your form, which will facilitate increasing speed.
I didn't even talk about track workouts, tempo runs, and fartlek training, the three most traditional ways to increase speed. My goal here was to provide some not-so-traditional suggestions.
As always, be careful not to overtrain, i.e., work too hard too often, and don't do anything you are not properly conditioned to do. Keep in mind your age, experience, and current fitness level. Also, you wouldn't want to try to accommodate all of the ideas at once, and you should consider where you are in your training cycle before adding some of these ideas. If you are in the middle of your racing season, it may be best to wait until after the season and incorporate some of these ideas into your next macro running cycle. Then look for increased speed next season.