Never use black on a dark background. Avoid using red and green next to one another. Inspecting a slide up close for legibility can be very misleading, e.g. using a slideviewer. If you can hold your slide at arms length against a bright background and read it, then it should be readable to the audience. If, however, you cannot read your slide there is little chance the audience, especially in the rear of the room, will be able to read it.
Typefaces easiest to read are bold, sans serif and solid.
Limit message to seven lines or less. Do not use more than seven words per line. Hold slide as it reads correctly to you. Write sequence number on lower left corner of the mount. Do not affix tape of any kind to your slides because this could cause the projector to jam.