There is no law that says small firms can do business only with other small firms. If you can get your foot in the door, working for Fortune 500 companies is the smart way to grow a profitable marketing firm.
This article is about one small company—my own business, Articulate Marketing—and how we have found ways to turn our small size into a competitive advantage. We now work with giants: Hewlett-Packard, Microsoft, VeriSign, Symantec, and NetJets.
What follows, below, is what we've learned working with big clients.
Big is beautiful
Big companies are good clients. They do things that help small businesses. For example, they pay on time. They understand marketing, so they brief well and don't expect the impossible.
If you have ever lost sleep over a small business CEO who wants you to take away all his marketing pain but can't say what he wants (or what he's willing to pay for it), then you know what I mean. 
There are collateral benefits too. For example, big-name clients add luster to your own marketing. Also, you can learn a lot from working with the professionals in world-class companies.
Small companies can help
All my clients work with multinational advertising, marketing communications, and PR agencies, all of which have scale and consistency on their side. But they can be slow-moving, bureaucratic, and expensive. Without trying to replace them, small companies can provide more responsive service in niche areas (in our case, copywriting and advice).
Apply your marketing skills to your own business. Polish your expertise like a diamond so that everyone can see it the way you do. Conversely, don't try to be all things to all people. An old editor of mine said, "You can't be a good writer on every topic under the sun. Pick one field and be the very best."
What they see is what they get
When you're a small company, what your clients see is what they get. If you're the person turning up for the pitch and you're the person doing the work, there are no communications barriers.
Fred Brooks writes in his The Mythical Man-Month that you don't necessarily get more productive by adding more people; you just increase the amount of time wasted on communications. Understanding this point is central to outcompeting your bigger rivals.
This time, it's personal
It's easy to see multinationals as impersonal monoliths. Wrong! To misquote Soylent Green, big companies are made of people.
Try to see the world from their perspective. People in big-company marketing departments are generally budget-rich but time poor. They like dealing with people who make their lives easier by delivering good-quality work on time without lots of handholding. As with the rest of us, they like doing business with people they like.
So your first objective is to make contact with those people in large corporations who can become your champion.
Make contact with the mothership
There are several ways to make first contact. You can go work for a big company and then leave to set up on your own. You can work for an agency that is already rostered either as a subcontractor or as an employee.
In my case, I made initial contact with some clients during my days as a freelance technology journalist and with others by word of mouth and recommendation, and when people move jobs. I have also had some success with invitation-only seminars. They give me a chance to share my expertise and begin a conversation. I recommend David Maister's book, The Trusted Advisor, which is packed with insights about this approach to selling. 
Be part of the family
Your objective is to get rostered. Once you are on the roster of approved suppliers, the nice people in the marketing department can give you work simply by raising a purchase order. You just need to concentrate on building a good relationship with them. 
Avoid the trap of subcontracting for a larger agency that is already rostered. It might seem like a simple solution to a big-company employee, but it won't do you any good. Agency subcontractors are the battered wives of the marketing industry. Every time we have been in this position, the agency has taken all the credit, marked up our prices, and made our life hell. We still have the bruises and scars.
Get past the purchasing department
In your quest to get rostered, your enemy is the purchasing department. They hate small companies. In their dreams, they have to deal only with one supplier and they get a 99% discount. In my experience, you need a powerful patron to strengthen your negotiating position with them.
The more unique or specialist or niche your services, the harder it is for a purchasing department to haggle about prices or play you off against other providers. If you have to give a discount, try to do on a pre-defined portion of your business, such as the first 10,000 words or the first campaign. Don't give away your profit margin in perpetuity. Try to avoid giving a daily or hourly rate, as this is easily negotiated away.
We charge a fixed rate per word so that we can shift a negotiation onto the scope of the work rather than the price. For example, if a client wants a 10% discount, we suggest a 10% reduction in the length of the copy. If you can, make them sacrifice something they care about to get a discount.
This is the hardest part of the relationship but it is also the time when your stock is highest with your new client. Know that you have leverage and that you can negotiate. You have my permission to say "no" to a bad deal.
There are downsides
Sometimes, big companies will behave like, well, big companies. The left hand doesn't always know what the right hand is doing. Office politics sometimes gets in the way. Some projects will take months when you could do it yourself in a few days if you didn't have to build a consensus first. Head office will kill projects at random. People will move jobs. They might get bored of you. You might get derostered arbitrarily.
All those things have happened to us, yet we survived. You will too. 
Remember that your loyalty is to the people you work with and that your job is to help them do their job without making life more difficult than it already is. Deal well with adversity and you'll get more work. Nobody likes a crybaby.
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We live in the Jurassic Age of big business. Giant multinationals roam the plains while midmarket raptors feed on them, hoping to grow fat. But deep in the forest, small, intelligent, warm-blooded creatures are finding their niche and waiting for the asteroid. One day, you will inherit the Earth.
2. See "27 Proven Freelance Marketing Tips."
4. See "Six Steps to a Stress-Free Career."