Never forget to include a good description, credit to the author, source and the right license tag (and justify, e.g. with a quote, why this license tag is right) - See project scope.
Please don't data-dump
Not all resources are useful for Wikimedia projects. Examples of non-useful resources include: images without informational value (party pictures, minor artwork without informational purpose), very old media which may have errors of fact, content from biased points of view. See project scope.
Before you add a source link
Check if the website with the link has a fair amount of free licenced content with a useable licence.