I've often been tempted to add my 2 cents to a discussion simply so I'd know where to start reading the NEXT time I looked at a thread. There's a much simpler method of keeping track though:

Before the actual messages, there's a list of everyone's names. Simply click on the last name on the list. This hops you to the last message, but your browser will also interpret the click as following a link, and will change the colour of the name. The next time you are looking at a thread, just check to see the last name you've got in "followed-link" colouring. Click on the name right after that and start reading.