[Philip Lelyveld comment - these are good first-pass rules.]
Historically, when a user experience becomes inundated with ads, some kind of solution appears and it's almost never good for marketers (pop-up blockers, DVR, etc.).
We need to begin proactively thinking about how to build a sustainable ad revenue model that balances the demands of publishers, advertisers and consumers.
1. Make ad placement predictable.
2. Don't oversell ad inventory -- exercise restraint.
3. Keep performance a priority.
4. Consider the user's privacy.
5. Give consumers a choice.
See the full story here: http://adage.com/article/digitalnext/marketers-ruin-virtual-reality-ads/302878/