Sure, you can run around and grab the white and green ammo packages that dead enemies drop, or you can buy replenishing kits from merchants in town, but everyone likes free stuff, right?
This Destiny tip—that Kirk shows off above—will probably come in most handy for those of you planning on soloing a lot of the game, as you'll be doing a lot of shooting on your own against sometimes hundreds of enemies. Seriously, those firestrikes can be lengthy!
So if you want to get the most out of your ammunition count, it's as simple as depleting your reserves on both your primary and secondary weapons (though it looks like you can just get them low enough, not exactly zero, for this to work). Then, wait 30 seconds. Boom! You've got free ammo. Now you can run around and pick up all the looted ammo off the enemies you just emptied your guns into. If you played the beta you may have already discovered this trick, but it's still active and still useful. Pretty easy, huh?