The best position will be "hull down," meaning that only your turret is poking up over whatever you're hiding behind. (If you saw my post in the "who's the best tanker" thread, that whole engagement was won because of using a hill as a hull-down emplacement.) A good example of this is the hull-down positions scattered around vehicle bases, however these put you at a severe disadvantage by limiting you to reversing in or out of them.
Sometimes this can be achieved by ramping part way up a hill or by creeping up to the edge of a steep drop-off, however you need to be careful of moving too far forward as that will expose the bottom of your tank to fire. The tank dies when the driver is killed, and as there's very little armor underneath a tank, it's very easy to put one right between his legs.
In the case of hiding behind a hill, if you're crafty enough, you can actually put yourself "broadside" to the enemy and point your frontal armor toward the direction they'll come around the hill, giving yourself ground cover on one side and armor on the other...optimally set up so that you can fire over the hill at him the whole time. This is much more difficult with the new hills, though.
In general, common sense applies. If you were going to take your fist and punch the armor from the angle that a round would hit, and your fist would end up stopping (and breaking a few knuckles), then you have a very high chance of a round penetrating. But if it were a position where your fist would skid or skip off the surface of the armor, the shell is likely to do the same thing.
