It likely depends on what kind of B2B service you are promoting, the size of the companies you are targeting, and how deeply your service needs to integrate with the target customers' existing software systems. In my experience marketing to businesses via the same channels that work well for consumer web products is no easy task.

Give serious consideration to making a real effort to discover, meet, interview, get to know, perhaps even partner with a talented salesperson with great personal and/or professional connections in the industry you're trying to reach.

Here are a couple posts that you might find useful:

  http://www.gabrielweinberg.com/blog/2010/09/sean-murphy-on-the-first-1-6-enterprise-customers.html

  http://www.walkersands.com/Comm_Selling_to_CIOs.htm