There is a cool feature in AdWords that allows you to restrict your ad to specific positions. This comes in handy if you know that a certain position, say position #3, has had the best performance historically. Here is how to set up Ad Positioning:
In the Campaign Settings, under Position Preference you can turn on the ability to mange your position preference and have AdWords automatically manage maximum CPC bids to target a preferred position range.
When you edit keywords in the Keywords tab, you will now have the option to select positions you’d like to restrict your ad to. It will let you select one position (i.e. #3) or a range of positions (i.e. #1 – #4).
Keep in mind that if according to Google’s algorithm your ad should show up in position #4 and you set your ads to display in position #3, your ad will not display at all. Use it when you have the data to back it up, when you know that any clicks from other positions will not bring enough conversions.