from my experience below is sequence of performing ad units.
- ad after question, just before answers.
- ad in sidebar
- ad after menu navigation bar, just after horizontal line. (This should be adsense links unit or similar)
- ad after all answers, just before related questions.
- ad after all questions.
also if you use too many ad unit then people may not like your site. They feel it is SPAM site.
Here is Q2A plugin for ads.
Demo of this plugin is here http://demo.question2answer.info