Archiving Gmail messages feels like deleting them, but they'll be around if you need them in the future.
If you're throwing your Gmail messages in the trash, you need to stop. And if you're doing nothing and leaving your messages in your cluttered inbox, you need to start doing something.
You should start archiving messages. Google says:
Archiving lets you tidy up your inbox by moving messages from your inbox into your All Mail label, so you don't have to delete anything. It's like moving something into a filing cabinet for safekeeping, rather than putting it in the trash can. This will keep your inbox clean and help you reference your archived mail more easily. You can learn more here.
I've read that archiving every message is the way to go. Enough storage space exists for you to follow that rule. But, as Dr. Ian Malcolm would agree, the ability to do something doesn't make it a good idea. So let's consider an alternative.
You should save anything that you might use or reference in the future. I mean anything. If a message has a greater than zero percent chance that you'll want to keep it, then keep it. For example, I archive every conversation I have with my family members, while I delete all marketing emails, like Groupons.