*** Race at an ambient temperature of no more than 16 celcius. For each degree above that you will lose 3 seconds over 5k. So that means that you are most likely to be able to plan for a PB in Spring or Autumn.
All the suggestions are starred:
*** 3 stars. IMHO it will work
** 2 stars. It might work
* 1 star. Can’t hurt
X 1 cross. It CAN hurt if done wrong.
All 50 tips are shown here as a series of posts : http://the5krunner.com/tag/how-to-get-a-5k-pb/ or on twitter using #howtogeta5kpb. They are all SHORT-TERM tips that will work in the next week or so with varying degrees of success.