I find the most effective way to separate joints is to use a side-lever separator, which gives you mechanical advantage and avoids the risk of damage that comes from flailing around with a hammer. As Burnerman says, you can get them from Laser Tools, who do a very useful range, also from such tool manufacturers as Sealey and Sykes Pickavant. Generally they come in different sizes, suitable for cars and vans/larger vehicles/big trucks.
I find it's also useful to have a separator fork: you insert the tines in the joint and strike it with a hammer. This can be handy if access is difficult, but it destroys the rubber boot of the joint meaning you have to replace it.