My strategy (for schools that are amenable to update letters) is to spread out my (significant) updates over different letters. You write one letter with a clear and focused thesis, say, an organization you have been involved in founding and running. You talk about that activity for the entire letter and perhaps how it fits with the school. A month or a few weeks later you write update letter 2: Maybe about the research that you've been doing during the application season and the publication that has been recently submitted. etc. etc.