Keep it simple and consistent.
PowerPoint has a number of built-in themes and templates that have coordinated colors, fonts, and special effects. PowerPoint's templates are a point of debate among experts (because so many are overused), but they all agree that consistency is essential. Having a theme is important, so try finding or creating your own templates. If you do use one, just be sure to choose it before inputting your content, otherwise things may become shifted around.
While background images can be useful in business presentations when you’re trying to impress investors, simple backgrounds are often best if you’re trying to instruct. If you decide to use a photo as your background (hi-res— you don't want a pixellated slide!), make sure it's not too busy. If text is going to go overlay it, just make sure it's either a big spot of one color, or use a semi-transparent text box.
An expert opinion is that cool colors are good for backgrounds, and warmer ones for text. One presentation complaint is of too many colors in slides; it’s best to be subtle and elegant with your background theme. Try using a palette of a few key colors. And be sure not to blind your audience with color. If your presentation is going to be displayed on a projector screen or TV with the lights dimmed, it’s often best to use dark backgrounds, so your audience isn’t blinded.
Make them easy to read and contrasted.
Your font and size will likely depend on the subject, room size, and if it’s displayed on a lit screen. You should always use fonts that are easy to read at a glance. Try Sans Serif fonts (they don’t have the tails on the letters). Only use artistic fonts if appropriate to the subject, the feel of your class, and your professor’s preferences. Just make sure they’re easily readable. Try not to use more than two complimentary fonts.
Make sure your fonts are large enough that someone in the back row can read it. And the font colors should also contrast with your background. So if you use a dark background, it’s important to make sure to use a white or light font, and vice versa.
Use these carefully.
Transitions are animated ways of transitioning to the next slide. They are attention-grabbing, but can also be distracting and out of place. If you do use them, you should not use multiple types of transitions for a presentation, and try to limit them; don’t place them between every slide.