Archive your emails instead of deleting them. Then you can always access them with the "All mail" link. If you send your emails to the trash, they will be removed after a period of 30 days.
On a different note, if you don't trust the cloud fully, I recommend taking a backup of your email and storing it locally. Then you can always access them no matter what happens to your Google account or if you accidentally delete important emails. One way of taking a local backup is using Mozilla Thunderbird and a plug-in that can save local copies of your emails in a standardized format. (Another benefit of that is that it becomes easier to search in your collection of email. Ironically, the search feature of Gmail has serious flaws.)