[Edit] a common term we used to use for a fully exposed tank was (semi-vulgar term for breasts starting with the letter 'T')-Up, but this is bleeped on TMP, so I'm changing it to "exposed"
Forgive my blathering, but hull down is an important subject to my mind. 
Of course, I spent most of my military career trying to achieve such positions!
Just find a small rise of ground to get behind. No need to depress your gun at all since you're not ON the rise, you're behind it.
There is very rarely any such small rise of ground on a relatively horizontal surface. In fact, the only time you can rely on such a thing is if you have a dozer come and plough a hull down position for you. Indeed, I have seen officer cadets fail an evaluation in the armour school when they did actually find such a rise and got behind it; there's no avenue to back down away off the fire position, you are always exposed!
There's much, much more to fighting a tank than getting behind covering ground. You have to be able to move off your position which means, ideally, to be able to back out of sight, jockey left or right and come up into another fire position. The enemy sees you disappearing to re-appear suddenly, left or right (I won't get into what role the turret down position plays in such manoeuvring).
I commanded a Leopard I and I recall how many times a seemingly good position was just a little too steep for me to bring my gun low enough to cover the ground in front of me. Being able to pick a fire position is an art. A trained crew commander is always scanning the ground not only for enemy, but for his next position if he is advancing to contact.
If I were into super detail (which I am not), I would roll for each tank where the player indicates it is taking a hull down position to see whether or not the position is successful, ie, provides cover to the hull, allows the gun to depress enough to cover the area designated, is the bore clear of the ground in front of it. The more experienced the crew, the better the chance. Depression limits of the main armament would also figure into the roll. A failure at the roll means the tank has come too far forward or is forced to move forward (to lower the gun) and is completely exposed.
I've been on many exercises where I watch the main force take hull downs against my position – the less experienced commanders and gunners (it is the gunner that guides the driver and tells him to "HALT!" when the gunner can see the desired terrain through his sight) really frequently screw up. They either do not have the gun clear, or they've come too far forward and are completely exposed
To make it simpler, I might simply indicate that some tanks, with very low depression limits, cannot assume a hull down, but this, as Allen mentioned, depends on the range at which you are firing.
Hull down is relative Remember too, that that hull down is always relative. One of the best wargame examples I've seen of explaining this was in Frank Chadwick's first edition of Command Decision, I think (it's been nearly 25 years since I read it, so I could be mistaken). When you assume a hull down, you usually come up the gentle side of a slope and you stop when the gunner can see targets. This means that you will not see stuff in the ground that is much lower than you unless you move forward some, in which case, you become fully exposed to the enemy on elevations equivalent to yours, but then are hull down to the enemy in lower ground.
In this picture of one of my wargaming tables:
picture
There is a long hill on the side of the table labelled "Soviets" and a "Church Hill" (I like the model train church I have and almost all the games I put on have a "Church Hill" ). Say a side is approaching from the Soviet side and takes up hull down positions on the long hill. I make my players specify "hull down to what?" In this case, it will initially be "to the top of church hill", in which case any enemy in the flat, low ground between Church Hill and the long hill will not be seen by tanks set up in such a hull down position.
Rolling Terrain As Wartopia and some others have indicated, there are terrain features and lay of the land that are very difficult to represent on a flat table with hills that we typically use for wargaming purposes. Yet this sort of configuration of real geography is vitally important in how tanks (and infantry) move themselves on the battlefield..
Because of that difficulty, for all intents and purposes, I tend to use small hill models that are sometimes so inconsequential that they are pretty much the "a small rise of ground to get behind" that donlowry mentions. You could always use what the Crossfire Hit the Dirt scenario book calls "ridges", and use a piece of string or some more aesthetically pleasing equivalent, to indicate many different places where the sloping ground changes from going up to down, behind which hull downs can be taken.
Models on Hills Remember what I said about the difficulty sometimes in successfully achieving a hull down? In this picture from the same game of the table picture above, a bunch of IS-2s are in a "hull down" position on the long hill, against Church Hill (on which there is a Panther or two) out of view, to the far right:
picture
Anyone can look at that and laugh at the ridiculousness of the appearance of these 1:72 scale tanks practically pointing at the sky! 
But this is (yet another) model to terrain scale relationship limitation of terrain modeling and 1:72 wargaming. If I were to make a hill gentle enough to put the IS-2s on a really gentle slop, I'd have one hill that would probably be wider than my 5 foot wide ping pong table.
So that's another thing to keep in mind.
--
Tim
