First and foremost, if you can, donate money to the American Red Cross or the Houston Food Bank. Consider donating blood as well, as blood banks are always in short supply and with what amounts to the arrival of an entire city of refugees, many of whom will need medical care, Houston supplies will be strained.

Locally, drives for bedding, blankets, clothes, food and other supplies are starting up. The Houston Chronicle has a list of drives and volunteer opportunities currently being conducted or getting started. Housing is also going to be necessary; the Astrodome is near capacity already. To that end, MoveOn.org has started a clearinghouse for people who have rooms to offer to disaster victims at HurricaneHousing.org. Apparently some evacuees will also be transported to arenas in Dallas and San Antonio.