There are two key factors crucial to assuring the success of any large SEM optimization process: First, tuning the campaign so that it is efficient and second, scaling the account while maintaining the efficiency you’ve gained. In any SEM account there will be a relatively small set of keywords that are naturally efficient (usually but not limited to branded terms) but that only drive limited volume.
http://searchengineland.com/two-golden-keys-to-paid-search-success-scale-and-efficiency-28421