You have to train those throw-ins in. It's not something you can just do naturally. You have to have at least 3 people (1 being the thrower) who know what's going on... It's the old "out and in".

The thrower has at least two options to throw the ball to, but sometimes no more so that the area doesn't get crowded. Both options should be marked (if not just throw it to them) but then one will literally run out of the space they're occupying. If the marker follows, another player simply runs into the space left, giving themselves either a yard or two on their marker or having no marker at all, leaving time to get get the ball under control and do something with it. If the marker doesn't follow the person running "out" then he's automatically free to throw the ball to.

It's not that difficult, just takes a bit of practice and understanding. The understanding is that if you're one of the first people to offer yourself to the thrower, it's likely that you're not going to be the one getting the ball. But it's likely to be you who's going to start the whole thing off and run out of the space you're in. If you're 2nd on the scene you're likely to be the one getting the ball or needing to run out of the space as well. If you're 3rd of 4th on the scene then you need to be aware not to get too close too soon cos you'll be running in to fill up the space left when 1 and 2 run out. If you're 5th on the scene then you're probably just getting in the way.