According to The Center for International Disaster Information (CIDI), which is part of the US Agency for International Development, donating money is almost always the best way to give aid. Before sending material goods (like blankets, food, or toys), the CIDI recommends confirming with relief organizations there is a real need for them. If you want to assist in person, nonprofits both international and local are looking for volunteers.
Here is a full list of reputable Puerto Rico-based charity organizations to donate to, followed by larger nonprofits that operate on a national or global scale.
Be sure to check with Charity Navigator to see if the ones you choose are highly rated and will spend 100% of donations received on hurricane relief and associated expenses.