If you are willing to buy an H, K, or Y fare it's not impossible at all. Unfortunately, if seats in the upgradeable business fare class are not available at time of booking (and there's a very good chance they won't be more than 30 days in advance), then you have to waitlist. People have generally had pretty good success clearing the waitlist, but it's no guarantee. Elite status does help here.

In short, not impossible at all, just annoying enough that many here (including myself), won't play the HoKeY lottery.

Now, if you mean free upgrades, then yeah, that's nearly impossible. Only way it can happen is if coach is oversold.

If the route is international and there is no BusinessFirst cabin (most of Central America and the Carribean), than the usual EUA rules apply and you can get upgraded.

Isn't it fun being a CO elite?