Hate non-design template hacks?

[Bilderback]

I do - cant stand them, especially when someone says "Add this code..." right after a template hook.
In the past, I have hacked alot of templates but after countless hours of updating them after upgrades,
I have decided to go anti-template hack.
Can anyone point me in the right direction so I can learn how to hook properly for a simple eval(template) ?
For example, I saw a template hack that wants you to add Google adsense code to your header template right after $ad_location[ad_header_logo]
What would be the proper way to simply hook that location via plugin to a Google adsense template instead?
Also, in order to hook certain $ad locations, would I hook global_start?
I just need direction please, so I can kick the template hacking habit.