Why not encourage your incumbent firms, the ones you are comfortable with, to cross-sell into areas of service where they have not served you (See my post of Feb. 20, 2009: cross-selling by law firm partners with 7 references.)?
One way to encourage firms to expand their scope of services is for them to buy entrée with discounted rates. Language in your terms of engagement to effectuate that might include the following: “As an inducement for the Company to expand the range of legal services performed by the Firm, an additional five percent (5%) discount will apply to the fees generated by each of the following practices, provided that the amount of the fees generated by such practice exceeds $200,000 during the following 12-month period: Environmental or Bankruptcy.”
Both the firm and the legal department acknowledge the possibility of additional work and put a price tag on the law firm’s earning that right.