The most efficient way to prioritize projects and complete the work is one item at a time. See the below diagram for further explanation:
This diagram was borrowed from the book “Scrum” by Jeff Sutherland (which I highly recommend). The explanation is as follows:
1.) The top line shows that traditional project management follows the line of thinking that everything is important so work on every project at once. By working multiple things at once, you lose a % of time to context switching i.e. waste. For example, working on three projects at once means you lose 40% purely to waste and only leaves 20% per project. (Scrum pg. 91).
2.) The bottom line shows that Agile says to focus only on one project at a time. You do not incur waste due to context switching and you finish all three projects sooner than focusing on all three at once.
Strategy: Focus on as few items as possible. This goes along with WIP limits. As well, it may be necessary to enforce hard project limits per person i.e. no more than two projects at any given time.