As james says the metal and how thick would be useful but some standard things apply no matter what,

1) Ensure that your drill bits are sharp. It is best to use high quality ones as these will require sharpening less often.

2) Make sure that the hole will drilled where you want it. Sounds a bit obvious but a few mm here and there can put everything out of place.

3) Use clamps to secure the metal to the plate below the drill bit. Believe me (speaking from experience here) holding it in your hand may seem like a great time saver but in the long run when you have to go find some bandages and clean up the red stuff on the drill etc it really isnt.