Juzzy,
One way to make yourself look older is to grow facial hair. Growing facial hair kinda hides your kid face and makes you look at least 5 years older then what you really are. You can grow facial hair faster if you shave it. Now I bet you thinking "How does shaving it off help it grow faster?" Now I'm not quite sure on this one, but for some reason, it does make it grow faster if you shave it. I could probably grow a full beard in about 2 weeks tops.
Another thing that makes you look older is your cloths. Don't walk around wearing side ways hats and skate shoes like a skater kid or something. Just wear typical jeans, shirt, (Not DC shirts, or anything like those either) and some regular average shoes.. This makes you look more like an adult, as if you have a job, car, and live on your own.
One other way is your personality. When you go to buy the 18 or older Merchandise, act calm, know what you want before you walk up to the counter. Don't stand their away from the counter and stare at stuff, and then walk up to the counter and say "I'll haaaaave........some of those" or something a long those lines. 18 year olds don't do that. They walk in, know exactly what they want, and act like you have done it a million times. Do NOT hesitate, just ask for whatever your getting.
I'm 20 years old and have been buying cigarettes for nearly 3 years now and hardly EVER get ID'd. I also go to casinos and hardly ever get ID'd either and believe me, I don't think I look 20 at all. A lot of people say I don't. The main reason why I don't get ID'd is because I just act casual and act like I'm just buying a pop or something you know? Don't act like your buying something you shouldn't be when you walk up to the counter, that is the main reason why people get ID'd. Believe me, I use to be a cashier before and have to sell cigarettes all day long to customers, and trust me, cashiers are trained on recognizing stuff like that too.
Another thing, don't go into a real popular store either, try to find one of those old stores that will pretty much sell you anything. A store that's ran down, and really not in to good of business. This will STRONGLY increase your chances then a real popular store like Walmart, Conoco, QuickTrip etc.
In conclusion, just pretty much act like your 18, and you know what your doing. Act like you have bought this item a million times and don't think anything about it or act like you don't know what your doing. Also, if one store doesn't sell it to you, try another store. You will be surprised, it may take 50 attempts until you find the right store, but if it's something you REALLY want, go after it.
Good luck bud, and I hope this helps.