I want to do some free work to build a customer referral base. Can I write some of it off as a marketing expense?
On your personal taxes, you are never allowed to deduct the value of your time. Not for business purposes, not for charity, not for medical care, not for anything. You are allowed to deduct only money or physical property.
However, if you have out of pocket expenses relating to the free work, such as electricity, automobile usage, long distance telephone calls, office supplies, etc., then you could deduct that, but only if this is really an effort to get business and make money, and not just a hobby.
If your business is a C corporation, then you could have it (the business) pay you (the individual), in which case the C corporation would claim an expense on its corporate return and you would claim wage or salary income on your person return.
No, you can't write off your time - there's nothing to write off, it isn't claimed as income either. If you have out of pocket expenses you can generally deduct those.
Building a customer referral base: that's a marvelous idea