I want to start a full service marketing agency. What are some tips on getting clients?

First, talk to people you already know about what you're starting and how you can solve problems for their business. The key is to show how you can help them. It's not about your business, it's about theirs. Pitch them some low-cost, low-risk solutions. The hardest thing to do is exceed the expectations of someone you already know but the quickest way to do that is partner/outsource with people that have skills you don't and use them for your early projects. Once you've exceeded expectations on a few projects, they will start telling other people in their networks. Boom, you're off and running.

*I have made my answer anonymous because I am publicity shy* The typical way to get clients for a agency startup is to pitch a kick-ass project to a company CEO or CMO, execute it in a kick-ass manner, then go back with another pitch, and execute that well, as well. After you've done this 2 or 3 times with the same company, you can broach the subject of pitching for the whole account. But even then, you won't be a "full service" agency because that is a whole other ball of wax. I say, choose one field you know well and go after that work first.

Starting brand-new I think you have to prove yourself to get into the larger companies. Start with smaller companies at a decent price to show what you can do. As you add clients you can increase your cost. If you Develop a model that your cost effective and productive, I think you have a great long-term business model

Capitalizing on what everyone has said, you cannot start a full service agency on the onset, unless you have loads of cash, talented people and contacts. Start with something you have expertise with, build on it then branch out as your revenues and billings come through.