The other week I went to a cool meetup about using EXPLAIN to look at MySQL queries to make them better.
Here's the presentation: http://files.meetup.com/3640402/ATT_2012-06-13_Optimizing_MySQL_Queries.pptx
Here's a cheat-sheet for EXPLAIN: http://files.meetup.com/3640402/MySQL%20Explain%20Cheat%20Sheet.pdf