There are some fundamental principles of design which you need to get familiar with. Once you do this, you can apply them to all sorts of situations. The first principle is UNITY.
This means very simply that your documents (or slides) should have the same look and feel all the way through. This can be achieved by using consistent fonts and colours and by not mixing up your graphics. For example, if you are using high quality photographs as visuals, don’t just suddenly plonk in some clip art because you can’t find an appropriate photo.
You might be thinking that this will make your work boring or that you don’t have time to fiddle with things to make them match, but UNITY is really important for your audience. It makes your audience feel comfortable without them even knowing why. It makes you look more credible and professional when your work has a unified look and feel.