Nancy Koehn, a professor at Harvard Business School, says, "So [one lesson in downturns] is market, market. Don't cut back on marketing."
You can read the whole interview here.
Of course, I'm not going to disagree. I'll keep telling anyone who'll listen that they have to keep talking to their customers, prospects, employees, etc. Even when business is in the toilet. Maybe not every element of marketing is important, but the key is to communicate.
A colleague of mine pointed out that she's seen public relations budgets staying more or less the same with advertising plummeting. Why? The later costs a whole lot more.
Keep talking to the people you need to reach. Keep feeding them relevant information and they will reward you. Of that, I'm certain.