Know about the room and conditions you'll be giving the presentation in (and test in there if you possibly can)
Make sure someone with okay-but-not-the-best eyesight can read/comprehend each slide quickly when they are sitting at the BACK of the room. You're testing font legibility, graphic size, and colour contrast. Particularly if the room itself will be bright or (heaven forbid) has a light source behind the screen.
No point in having a brilliant slide if they can't read a word of it 🙂