One approach that some of these firms are already taking, is to harvest "assets" out of their services engagements. The idea is to capture and reuse the work done for a specific customer, with other customers in similar situations / domains. The assets could be software, design documentation, scripting, processes / workflows, etc.
Another approach is to bring in expertise from other parts of the company, at higher rates. For example, IBM has this programme where experts from IBM Research spend time on customer projects, billable at much higher rates than the usual. Naturally the customer expectation is that these researchers are adding significant value in problem solving, optimization, etc.