Basic principles for online and multimedia learning
Here's a quick glossary of your most critical multimedia design principles for learning according to the research literature.
Avoid using irrelevant video, graphics, music, stories, narration, etc. The less your learners know about what you're trying to teach, the easier it is for them to waste time and effort paying attention to anything you show them that is not directly relevant to the lesson. When you're teaching learners with greater prior knowledge, however, some motivational imagery may increase their interest and learning effectiveness just a bit.
Keep related pieces of information together. Show graphics and play the narration that describes the graphics at the same time. Or if you're using text, place the text that describes the graphics right next to the relevant part of the graphics. Display your feedback along side of the answers that the student provided.
Some of the things that help low-prior knowledge learners learn best (such as delivering lessons in short segments, giving learners control of pacing, use of pre-training, and others) may have no beneficial effect on high-prior knowledge learners. Some of these practices may actual reduce your lesson's effectiveness for these learners.
Learners tend to do best when you stop the narration and have them click a "continue" button in order to start the next segment. Research suggests not overwhelming the learner with too many control options, however, so keep it simple. Also your high prior-knowledge learners may learn better when your lessons move forward automatically, but they have a pause button that allows them to stop when they choose to do so.
Generally speaking, audio narration leads to better learning than the same words presented as text on the screen. This is especially true for walking someone through graphics on the screen, and when the material to be learned is complex or the terminology being used is already understood by the student (otherwise see "pre-training"). One exception to this is when the learner will be using the information as a reference and will need to look back to it again and again (like this glossary, for example).
Relevant graphics, audio narration, or explanatory text... combining any two out of these three different ways to deliver information works better than using just one way or all three.
Use a first or second person conversational style in text and audio narrations. Your learner should feel like someone is talking directly to them when they hear your narration. Also adding pedagogical agents (computer characters) can help if used smartly. For example, have them narrate the lesson, point out critical features in your on-screen graphics, or visually demonstrate concepts to your learner.
Teach important concepts, facts, or new terminology before getting to the heart of the lesson. If your learners don't understand the difference between a "dividend" and a "divisor" before they start your lesson on arithmetic division, then they're not going to get very much out of your lesson.
Don't narrate onscreen text when also showing relevant graphics. You're basically asking the learner to divide their visual attention between reading the text and viewing the graphics.
Break down long lessons into several shorter lessons.  Break down long text passages into multiple shorter ones. A series of 5-10 minute videos, each focusing on one or two related topics, is better than an hour long video covering a dozen related topics. People learn best when they can stop and think about what they just read/heard/watched. Also, it's easier to keep someone's undivided attention for 5 minutes than for a full hour.
Using visual, auditory, or temporal cues to send the message that something is particularly important for the learner to remember. Examples include highlighting, pointing, verbal emphasis in narration, and pausing for emphasis.