You can donate online at http://www.redcross.org . Note that many employers will match employee donations -- check to see if yours will.

By far the best thing you do is give cash. This can be directly translated to what they need. Collections of items are not as good, as they have to be shipped and sorted, and might not be needed. If you want to personally volunteer, you should contact the Red Cross or other agency first before traveling to the area.