I have introduced some new people to MySQL recently and had to back track the years to figure out how I learned what I learned about MySQL indexes.  A quick way to get up to speed about MySQL indexes is these three podcasts by Sheeri Cabral.
  1. OurSQL Episode 13: The Nitty Gritty of Indexes
  2. OurSQL Episode 17: Hashing it out
  3. OurSQL Episode 18: De-myth-tifying Indexes
Those three episodes do a good job of explaining how indexes work so that you have a better understanding of how MySQL indexes find your data.