Save copies of what you submit and publish.
Backup your work. Nothing lasts forever
When you submit a story to an electronic publication and they accept and publish it, never expect your story to be forever accessible. Anything could happen that might cause your story to disappear. The most likely scenario is the publication your story appeared in ceases to exist. More times than not, they will not only become defunct but so will your story. Either the domain will disappear or change owners and focus. The story you so diligently worked on and submitted will vanish into the digital ether.
Keep a record of where you submit and get published
The same is true for any social networks or writing communities you are a member of. If you have a page you actively post to, keep a copy of everything you post or publish to it. Note the site, the dates posted/published, and any comments and responses. These items will prove useful if you decide to submit your story somewhere else. Though you may not have any traceable evidence your story saw the light of day, you will have data to corroborate your claim you published your story before if you are trying to sell reprint rights. And that just might go a long way in convincing a potential publisher that your story was published before.
Use the Cloud and maybe a few other locations
Ensure your backup doesn’t get corrupted, save a few copies of your work. Archive your stories on flash drives, terabyte drives, or cloud storage like Google Drive, iCloud, One Drive, or DropBox; a hardcopy also helps. If the work is extensive and you don’t want to have to retype it again, scan the hardcopy and save it in a manipulatable format like MS Word. This way, if something happens to one (or two) of your backups, you will still have an archive of your work. It’s highly unlikely all of your archived copies would be lost.