So you just bought a pack of cigarettes and you don't like how your cigs come, packed in to their box in tighter formation than British soldiers during the American Revolution. You don't want a weak smoke, that's why you bought a pack, so what to do? A common practice (In America at least, I haven't been outside of the country recently) for before the opening of a new pack of smokes is to pack them down. I hope this doesn't get confusing. It shouldn't be.
Packing your pack is done to condense the tobacco in its little paper cylinder. With each puff of a packed cigarette, one would receive thicker smoke, and presumably, a better "buzz" from your smoke. While not the most important thing in the world, an un-packed smoke will taste the same as a packed one, they'll just burn a little bit differently, there are a few things to remember.
First off, I will assume the pack about to be packed is a hard pack/box of smokes, and not any of the softpack booshizzle. Forget that noise, softpacks are silly. They're also not good for packing because one miss hit will cause the end of your smokes to crumple, deteriorating the aesthetic quality of the tip of the cig. It could also cause a cig to break, which would be highly unfortunate.
As always, you shouldn't even have bought this pack to begin with, but we'll assume you're addicted. Before tearing through the packaging to the tobaccoey goodness, pause. Turn the pack upside down, so that the part of the box that opens faces the floor/ground. Curl your pointer-finger, then grab about one inch from the end that's pointing to the ground with your thumb and curled finger. This is approximately equal to where the flip-down top meets the box again. And in reality, exactly where you grab doesn't matter.
Now the fun part, to slap the pack silly! Not quite, but, with our gripped pack in hand, we're going to want to pack it down. Slam the box against the heel part of your other hand. You know, the chubby part of your hand just above your wrist, by your thumb, oh yeah, right there. Repetitive smacking will cause the tobacco to be packed tighter in their little cigarrette shells. And you could go blind or get hair on your palms :^)
If done correctly, your cigarettes should have some of the paper cylinder past the tobacco, making the cig look like the filter end of a Parliament, recessed. Pack to personal preference. Note well, if you are just getting back into smoking, or were upset and grabbed a pack after a shitty day and usually don't smoke, don't pack them too much! The tighter the pack-job, the less air that you need to live will get into your lungs with the smoke. This can cause shortness of breath and totally roX0r your throat. As always, quit smoking now, because it will kill you, even though it is fun and useful in social situations. Or just smoke weed, because you at least get high with your addiction.
Packing is not usually done with packs of clove cigarettes or bidis or any other tobacco product. Also, don't roll a joint then put it in a pack to be packed, it doesn't really work like that.
Notes from Noders:
Besides pointing out my bad grammar and hideous spelling, mirv notes that packing would also be done to prevent any loose tobacco from falling out of your cigarette, etc. gnarl suggests placing the pack in the corner of a window sill and slamming the sill down on said pack, which would work, but sounds a bit more dangerous than your hand, to me anyways. Also also wick notifies me that most of the good smokes, the expensive ones like Dunhills shouldn't need packing, as they come pre-packed.