Well there are a few things you can do
These are things i have found from my experiance...
Set the engine to have a few more iterations (10 seems good)
You can do this by setting (PhysicsSimulator.Iterations = 10;)
Fiddiling with the values of the engine itself
Done like this
PhysicsSimulator.BiasFactor = 0.3f;
PhysicsSimulator.MaxContactsToDetect = 4;
PhysicsSimulator.MaxContactsToResolve = 2;
PhysicsSimulator.AllowedPenetration = 0.1f;
Now then there is the thing that will work the best.. But it is also the hardest to get working
It is the interactivity controller and it can also make you simulation run much much faster when things are resting. But it takes ALOT of tweaking. This is how you start.
PhysicsSimulator.InactivityController.ActivationDistance = 200;
PhysicsSimulator.InactivityController.MaxIdleTime = 500;
PhysicsSimulator.InactivityController.Enabled = true;
These settings are what i use they should get you going also make sure to set activation distance a bit larger than you largest body in the game.
Also try to (if you can) keep your stacks under 8 or 9 high
Hope all this helps in getting you started.