With Harvey's devastation and now Irma looming, the public will be motivated (rightfully so) to give whatever they can to assist those affected. The best way to help is monetary donations to an organization active in disaster relief, with a dedicated fund set up for the express purpose of assisting survivors. I can provide recommendations, but people tend to have strong reactions so all that aside, just do your research!
In-kind donations, or goods/materials, are almost never a good idea. They present logistical issues with packaging, shipping, sorting, delivery, etc. and often present a greater financial burden than what they would offset. Cities/organizations often end up with more material than what is needed, or a random smattering of secondhand goods, such that managing these donations has been termed "the disaster after the disaster." It draws vital resources away from the relief effort, and toward mundane tasks of sorting, handling, and usually disposal.
Financial donations allow relief organizations to purchase according to the needs of the community, and to purchase WITHIN the community, which will help to kickstart long term economic recovery. Financial donations also help to pay for case workers, contractors, lawyers, mortgage counselors, and a whole array of other services involved in disaster recovery that are not often thought of as immediate needs.
If you must donate goods, do the following:
1) Coordinate with a local (to the disaster) entity, who is aware of present needs for goods. Decide who will pay for shipping and handling (news flash, it will almost always be you). 2) Collect new, packaged items that can be palletized easily (e.g. by the case or box). Hygiene items, baby supplies, socks, underwear, non-perishable foods, bottled water, etc. are all common needs. 3) Determine, with the aforementioned entity, who will be receiving these goods and where, and how they should be shipped (by pallet or as boxes). Follow instructions!
In essence, goods are difficult to manage and require equipment and people to move around. Financial donations are efficient, easier to manage and track, and can provide a wider array of assistance.