Even before you open Expression Blend 2 and start working, it is very very important to plan work the way you would plan any website. One thing that I have noticed while working in Blend is, once you have created something in it, it is very difficult to change in the later stage of development. So, be careful while you plan. Do not keep much scope for changes while planning.
Take this small example. Assume that you are expected to create a presentation in Blend 2. Begin with planning the stage/canvas size that could be completely displayed by all browsers at 1024x768 resolution without much scroll. This canvas will contain all your other canvases in the hierarchy of elements. A canvas is nothing but a group of graphic or text elements.
You can use range of tools to create elements with different shapes or text.
Going back to planning, make sure you have planned the navigation of the presentation. How each slide or animation is going to come up in front of the user. How many scenes are there and how many slides each scene is going to contain. This is very important as this will help you determine the folder staructure and file breakup of your presentation.