Mass-delete old emails
As someone who only ever archived and never deleted a thing, I got uncomfortably close to maxing out my free 15MB of Gmail storage. But rather than spend hours combing through everything to delete the bulk, you can use advanced search filters to do some mass purging.
To delete message from before a certain date, plug in "before:YYYY/MM/DD" in your search bar. Click to select all, and an option appears across the top to "select all conversations that match this search." Then delete!
To delete messages that are taking up a lot of space (i.e., with lots of pictures or large attachments), try searching "larger:10M" to retrieve all messages larger than 10 million bytes. Delete those too!
There are dozens of different ways to filter things, so go wild. Just a heads up that the 15GB of free storage is shared between Drive, Photos, and Gmail, so if you're hoarding Google Docs or Photos you might want to clean those up, too. Or, if you're too lazy to deal with any of this, there's alway the option to pony up a cool $1.99 a month and upgrade to 100GB.