Homemade Stops

Wrap a bit of tape around the bit, to mark the desired depth. Then carefully drill your holes, stopping when you've reached the tape.

I've also seen this done with a bit of wire.

_{WARNING: Wrapping the wire in the wrong direction, can lead to property damage, injury, and death.}

Store Bought Stops

Or you could purchase a drill stop.

Which are also available in plastic

Portable Drilling Guide

There are also portable drilling guides available.

I tried one of these and it was total crap, so I wouldn't recommend it. Your mileage may vary though, so give it a try if you like.

Use oil to cool the bit, if oil residue is not a problem. Use water if that will be a problem. Keeping the bit cool is crucial to long bit life. The larger the hole the slower the RPM are to be used to drill the hole. Slow enough it is easy to count the revolutions is a good reference for how slow. When using a drill press the RPM are usually slower, and oil or water should be used there too. If the surface is flat, and water is preferred, an easy way to keep water at the drill tip is put an ice cube beside the hole while it is being drilled. The heat will melt the ice and keep a supply of water while drilling.

Another thing I do that I feel helps the large hole get drilled is using a smaller bit to pilot a hole through the metal first. Watch the last part of the hole just before the drill goes all the way through, the flutes of the drill bit will grab hard into the hole and try to wrench the drill out of your hands. That may hurt....