Mead is somewhat more resistant to oxidation than fruit-based wines. That said, I've heard of a few things you can do:
Potassium metabisulfite, if I remember correctly, has some anti-oxidation properties. I don't know that you'd want to rely on it alone, though.
You could top off the bottle with inert gas (nitrogen is the usual, I think). I think this is the most common method aside from stabilizing with potassium metabisulfite. I think there are even devices intended for preserving open bottles of wine that can be used as the nitrogen source.
Alternatively, I've heard of people putting sterilized glass beads of the sort you might get for decorative vases in the bottle to fill up the extra space.
Depending on the container, it might be possible to use a wine preservation device that creates a partial vacuum in the bottle. Again, there are widgets that do this to preserve bottles of wine after they've been open.
Lastly, you can move up to 5 to 5.5 gallon batches.
Since the exposed surface area to volume ratio is smaller, they won't oxidize as quickly.
There may be other methods I'm not aware of as well. Depending on just how much headroom you have, how long it's sitting before bottling, and whether there's actual fruit or fruit juice, it may not even be worth worrying about.
