Giving up's easy. I've done it loads of times.
A useful tip a friend gave me was to fill an empty jam jar with water and old ciggy butts. They really stink. Every time you feel the urge to light up, have a good sniff. Apparently makes you want to hurl and reminds you how your breath/clothes smell to others when you've been smoking. Works wonders, apparently, although I didn't try it myself. I echo the other commenters in that the best thing to do is just resolve to quit and do just that. Don't have a 'symbolic' last fag - these things are killing you so why make a mockery of a momentous moment? The important step is the decison to quit not the final desperate farewell to an old friend. Make sure everyone you know is made aware that you're stopping smoking (note - not 'trying' to stop) - if they care about you they'll help you, either by keeping an eye on you or not smoking in front of you (not like my mates who blew smoke in my face and kept offering me fags. Sods.) Make a note of the date you decided to quit and stick a note on the fridge door. Throw away all your ashtrays. Have bets with people that you never smoke for a week/month/year - keep extending it - everyone will be watching you like a hawk! Keep all the money you used to blow on baccy in a tin and watch how fast it grows. One your one year anniversary if you're a 20-a-day person you could buy yourself a whole new wardrobe, a round the world trip, a car, hell you'll be loaded - what about after 5 years?
I was a 20 a day man myself, which I know is small potatoes to some but trust me, it ain't that hard to stop, and after a while your clothes won't ming, your breath won't floor a horse, and even the smell of a fag will repel you. Then you can torment your (still smoking) mates and become a 'reformed smoker fag nazi' like me. Good luck - it's SO worth it!