As I ask (and answer) in my post at Search Engine People, how do you outsource marketing without losing your voice? Let's take design. How do you outsource design of your ads and marketing materials without losing consistency?
Managing an outsourced team can be no different (or difficult!) than managing a team in-house, provided you follow the rules.
For example, rule #4:
Give detailed, specific instructions. I can't tell you how many times our clients fail to do that and are surprised when our designs don't match their expectations. If you have something specific in mind, you have to really tell them what you mean. You can't just say "make it like our current logo, only better." You have to say, "I don't like this black box in our logo because it makes us seem unapproachable. Replace it with something pink and poufy and transparent." Yes, the designer will use their judgment: that's what you're paying them for. But you can't give them vague statements and expect them to read your mind.
Now go over and read the rest. Don't forget to tell me what you think I got right and what I missed.