Of course this is not always possible and is limiting on the film, but it is something to keep in mind if your film is destined for the internet. Avoid fancy transitions - Try and use straight cuts. Dissolves and fades increase the file size and as the codecs struggle to make sense of them look terrible. Have clean clear sound - Concentrate on making sure that the audio is compressed well. Balance the payoff between file size and quality.
Bad audio can ruin a good film.
Keep the audio simple. That way you are secure if lawyers start knocking on your door. Don't include credits - Small scrolling credits digitized from a video copy are impossible to read and unnecessarily bulk up the movie's file size. Instead make your final frame informative. Include your name, email address and the URL of your website.