Just drain it every time it's used,it will eventually work all the water out, but keep in mind everytime the compressor starts it's adding more water. An inline filter will help keep the water from tools.
One little tip: remove the drain valve and install a street ell in it's place and add a hose between it and the drain valve. this will make it easier to drain so maybe then it will get drained more often.
HTH,
Mike

Don't worry, every compressor is wet in the inside a little more water or less doesn't hurt. It had nothing to do with your stapler as it takes air from the top. Make sure to oil those air tools once in a while. Steve

This brings up a a good point. How does one know
when a tank has rusted bad enough to not be safe?
A friend had an old compressor blow up in his garage
and take a chunk out of a block wall. Luckily no
one was in the garage, as it could have been deadly.
From what I have seen, the way the horizontal tanks
are oriented is that the welded seam is on the bottom,
where all the water collects. That seam is where my
friends tank let loose. I know that a good weld is
stronger than the original metal, but how does this
play out when water is added to the equation?
Perhaps tank manufacturers should weld heavy straps
around the tanks to prevent catastrophic explosions.

You can sound the seams with a small hammer. Rusted soft spots will not ping but thud.
An air compressor can Make 1 gallon of water per horsepower per day. All that water does not condense in the tank. Much of it condenses in the hose and or tool and the rest passes thru as vapor. Oilling tools is the key. Not just at time of use but before storage oil em up. That bit of water sits in the tool and can ruin it while not being used.
There can be rust getting down the line as that is a common problem. Use a simple particulate filter to protect your tools a bit more.
A coating of rust actually is the only protection a non coated tank has. Remove it and it rust again right away.
Keep em drained and check them with a hammer from time to time.