Guidelines you should follow.
1. Use link units to maximize the CTR. Demo
2. Use Verdana Font in Ad Unit (For Text Only) so the font in your Google ads will show big title in ads and hence increases the chance of getting better CTR. Demo
3. Use 468*15 and 728*15 link unit on top of the page, Below the Title of Your Page and also Put 120*90 Ad Unit Right Above or Below the Labels or Achieves.
4. Based on study it is proven that Large Rectangle, Rectangle , Leader Board and Wide SkyKeeper can Generate more CTR.
5. If you are using link units then i suggest you to use 4links link unit instead of 5links link unit, because the font size of 4links link unit is bigger then 5links one.
6. Use Large Rectangle and Rectangle Ad unit within your post or below header.