What are some inexpensive ways to be more effective at marketing?

While not free of cost, a public art program’s website and online presence can be a marvelous tool for informing the public of new commissions or newsworthy events.  Maintaining a clear and detailed website also ensures that those who are interested in learning more about a public artwork have an accessible and informed way of doing so.
Likewise, direct staff engagement and outreach can be effective means of marketing to specific audiences or communities, and the cost of doing so is typically already covered under pre-existing staff expenses.
Partnering with client organizations can be an effective way of marketing a new commission that requires spare, or no, funding from the public art agency itself.  If a new commission is being made as a result of a significant building, expansion, or renovation, then it would be wise to pair news of that work with news of the new artwork, and so “piggy back” onto the press coverage.  Here, again, it is important to cultivate good relationships with client organizations in order to maximize the impact of news coverage. 
Finally, one of the best ways to generate interest in a new commission is to generate press coverage for it, but press is neither guaranteed nor automatic.  If your public art program, or the municipality that houses your program, employs a public information officer, then that individual will likely be the best option for assisting with and advising on press coverage and contact.  Otherwise, as a general rule, the press is only likely to cover new artworks and events if they generate public interest and so exercising some strategy on how an artwork is pitched or unveiled can help improve the press coverage it receives.