While lots of people have discussed how donated goods can cause problems in a relief setting, the fact remains that many groups will continue to send goods and there are some groups that can make use of some goods. This briefing paper by the "logistics cluster" in Haiti gives advice on how to appropriately send goods, not just explain what problems are caused by poor planning: PDF version (100kb) and the original link with summary at Relief Web.
A brief quote:
In order for humanitarian aid to be of the most benefit to affected populations, donations should be well-planned with national authorities and the humanitarian community coordinating the relief effort and fully compliant with national requirements for the importation of goods. Cargo that lacks documentation and adequate planning for onward delivery may have an adverse effect on the relief effort by taking up scarce resources, such as aircraft landing slots or storage space, and can place an additional logistics burden on organisations working on the ground.