Okay, I think I will add my thoughts.
You only live one time. Often, we spend too much of our lives lamenting what we didn't to, as opposed to what we've actually done. We all touch someone else's life, each time we come into contact with them. Even if it is something simple. So, don't try to be someone you're not (I know, irony on a webite with "handles"). Instead, be who you are, and let others worry about the rest. If you don't like yourself, how can anyone else truely like who you are?
The world isn't perfect. We, as flawed beings, are far from perfect ourselves. You can never be perfect. Instead, always try to be the best person you can. You're going to screw up. It happens. But your measure is not determined how you err, but how you react to said occurance.
Take one moment out of your day to do something nice. It might be to tell someone you care about you love them. It mght be to open a door for a stranger. Doesn't have to be big. In life, you are always repaid in kind. Not necessarily right away, but karma always evens things out.
---Now, back to fighting aliens and monsters.