Bonus: Avoid making these 3 mistakes!
There a few mistakes we've seen customers make frequently:
Wasting too much time on landing page experience.
Landing page problems are very rare these days, despite what Google may say. The Adwords diagnostics bubbles are not very reliable, here's an example: Notice the increase in quality score due to landing page experience going from average to above average. But here's the problem: we didn't change a single thing on our landing page (this one).
The increase in quality score for this keyword was accomplished by increasing our ad CTR alone. (Ad group structure had been taken care of, of course.)
Conclusion: Don't focus on your landing pages for quality Score optimization and don't always trust the AdWords diagnostics bubbles.
Trying to optimize all campaigns and ad groups at once.
It's fine (and recommended) to restructure everything that needs to be restructured at once, but when it comes to ad writing — which is the activity that yield the most results — focus is key.
Stopping all efforts with restructuring.
Some customers believe, in part because of our shortcomings, that restructuring is all it takes. Sometimes it is, often times it's not. Some customers don't even bother with restructuring their campaigns or ad groups. It is our dream that our software will someday do everything for you but for a short while, please follow the instructions if you want results.