I totally agree with all the comments above. Before you go anywhere near your computer you need to have a good story in mind, and sketching a storyboard with pencil and paper is a great idea. Keeping it short is also a great tip, your audience will appreciate a short animation that is engaging and well done more than a long video with a wandering storyline. Once you have your storyboard laid out STICK TO IT, don't be tempted to make changes once modelling/rigging is in place other than minor modelling or background tweaks, as they can very easily get you off track. $imon's tip regarding using temporary "block" elements is an excellent one, as this method allows you to "direct" your animation and iron out the wrinkles without being bogged down with slow rendering. Let's face it, it's the non-technical elements of an animation, such as the rhythm, pace, mood etc that are hardest to get right and are most likely to turn a good animation into an amazing one.