I currently manage an account that was previously run by two of the largest SEM houses in the industry.  This account was at one time bidding on roughly 17,000 keywords.  14,000 of these keywords were producing click charges and management fee charges, but resulting in ZERO conversions.  Out of these 17,000 keywords,  4 resulted in 90% of their sales.  Multiple attempts were made to explain this trend to those who were managing the account, but this fell on deaf ears. (while the management fees kept rising for clicks that produced zero sales)

Looking deeper into the account, these 4 keywords were producing roughly 4 conversions per day in their current account structure.

To my amazement, an account this large and an agency of this size was running only ONE campaign in Google!  That means those 4 money producing keywords were sharing the daily budget with all of the other keywords.

The first change we made to this account was to break out the account to multiple campaigns dedicating budget dollars to these specific keywords.  The result . . . the 4 main keywords are now producing roughly 30 conversions per day.  A 650% INCREASE IN CONVERSIONS!

Give high converting keywords the respect and budget they deserve, use multiple campaigns.  Google allows 25 campaigns per account and 100 ad groups per campaign.