You should always have your return on investment numbers in mind when bidding, but some find it useful to find a bid number they are comfortable with, and stick with it no matter what.

Your first pay per click keyword bid.
The advantage of this bid type is that you will always make money as you are bidding based off of how that keyword is performing.

The disadvantage is that once competitors realize you have a static bid, they may slightly outbid you and push your position further down, which will result in a lot less visitors.

In this instance, a slightly higher bid could lead to more visitors, more sales, and more profit, even if the actual ROI number is slightly lower.
These bids only change when your ROI changes, so there is little need of PPC bidding software.

However, as these numbers are very concrete based on how your keywords are performing, there is a great need to implement PPC tracking software to measure the effectiveness of each keyword for each pay per click search engine.

• Changing PPC Bids - by Position
• Changing PPC Bids - by Gaps
• Remember your content bids
• Web 2.0 Conference - Yahoo Speaks
• Pay Per Click Bidding - Gap Bidding