I actually learnt this great advice from a friend of mine.  We were visiting with her at her mother's place and there were a number of kids wanting to play with the same toy.  She got out a timer and told the kids that each person would have their turn but when the buzzer went off  they would have to switch.  It worked a charm and eventually they moved on to other toys.

We actually used this advice tonight with Mia and a friend who was visiting.  The two kids had been playing very well for a couple of hours when they both became fixated on the cash register.  We told them that they needed to take turns and to help they would have to listen for the buzzer.  The timer on the microwave was set for 2 minutes and once the buzzer went off they dutifully changed over (although Mia did need a little convincing at one point).  Within 10 minutes they had moved on to throwing lego around Mia's bedroom.

So when you are having trouble talking kids into taking turns use the timer it is awesome.