We picked up a new client a while back, adding capacity to a busy PR team. My experience in journalism has always been a handy tool for PR writing, but what has really wowed the client was the quick turn-around we offer. Now they're sending us short pieces that need a quick revision or copy edit in addition to the writing work.
The quality of the content was what got us the gig and what made them see we could help as editors, too. Executing that quality in a timely way is what convinced them to expand our scope of work.
Always deliver what you promised, of course, and look for opportunities -- tangible or intangible -- to add even more value. This is what separates you from everyone else on the team or in the marketplace.