Say you have three big projects on - we'll call them "The Big One", "Middleweight" and "Express" to illustrate the fact that they're of differing importance. Management have made decisions about available resources (or people as I often like to call them). It has been decided to dedicate 3 full time developers, 2 testers and one product owner to The Big One, 2 developers, 1 tester to Middleweight and 1 developer and a part time tester to Express. Middleweight and Express will share a product owner.
Why not just create a few simple coloured cards for these? 6 big red ones for The Big One, 3 big green ones and one small green ones for Middlewight and 1 big yellow one and 2 small yellow ones for Express. Write the project name in big letters and "Developer", "Tester" or "Product Manager" on them accordingly and place them in a marked out area on a table. When you allocate your staff, they take a card and pin it over their desk somewhere visible. No-one is allowed more than one big card or two small ones.
Now, take a look at the "holding pen" we created on the table: are there any cards in there still? If so, you know you're under-staffed and in what area. Look around the office: has anyone got more work on than they should, or is anyone seriously slacking? Smashing, everyone's got just the right amount of work on :)
Now, obviously, we have to manage other activities: the odd emergency here and there, process improvement, perhaps some personal learning and so on, and we should create temporary cards for these. So, for example, you have a Big Emergency to deal with on a legacy project or for a demo and you need to pull someone: grab a temporary wipe-clean card and write it on. Go pick your developer/tester/PM and they pin this card up. Obviously, any work they have on that takes them over their limit has to come down and go back to the holding pen.
Now you can see the effect that the emergency has had. When the PM head walks past the desk and sees a big red card in there, they immediately know that something's awry and can start asking questions. No cards in there and they're happy.