1. Smoking indoors will definately be the most prominent factor in lingering smells. If you smoke indoors, concealing the lingering odour is not impossible, but definately more difficult, for best results, smoke outdoors and do not hotbox. If you must smoke indoors, vent as much as possible, use odour control devices (as mentioned below) and use a sploof (container or tube with a tightly packed bundle of fabric softener sheets which is breathed through after taking a hit to dilute the cannabis smell with the odour of the fabric softener).
2. Ganja smoke will always leave a lingering smell in the area it was smoked for at least ten minutes, two to 20 hours if done entirely indoors, this time depends on strain, odour control devices, and general ventilation. Use a form of odour control, even brewing coffee or leaving vinegar out can be of benefit. Blow smoke out of windows while smoking if possible. Incense is very effective, depending on how much was smoked, how much incense is used, and how strong the smell of the incense is.
3. Alot of that cannabis smoke that you are breathing out attaches to your clothing, skin, and hair, which is why you will smell like pot afterwards. To destroy the smell entirely, do the following:
a. After smoking, go into the bathroom. Swig back some mouthwash or brush your teeth, or both, listorene mouthwash and minty toothpastes are very effective. Spit it out and wash out your mouth with water. Use chloroseptic on your throat (three sprays maximum needed).
b. Take a shower. Wash body with soap, wash hair with strong smelling shampoo (denorex recommended, has extremely minty smell) and a strong smelling conditioner (conditioner is optional). Get out of shower and dry thoroughly.
c. Use deoderant. Shave, if necessary. Use aftershave cologne (I prefer musk as it smells strong). Use additional face washes if desired.
d. Use eyedrops. Though not necessarily a smell problem, eyes are a dead giveaway if red or if pupils are somewhat overly dialated, though this problem varies from stoner to stoner and also how high one is.
e. Change into fresh clothes and drink as much water and eat as much food as you prefer. Spicy foods will further mask your breath, but the mouthwash should do the trick. Smoking, drinking coffee/tea will also aid in decreasing smell.
4. Finally, I want to make special mention that smoking out of a water pipe, especially one with lots of water and/or ice lowers the time period for which odour lingers a good deal when compared with a joint or even a spoon or other waterless smoking device. Smoking out of a water pipe with hot water, on the other hand, causes the smell to be quite strong when first exhaled, mind you, as hot water tends to evaporate a great deal of ganja's odour molecules I believe.
Other than that, all I can suggest is to not handle any cannabis after this and to wash your hands, hair, mouth, and clothes thoroughly as these are the most probable areas for odour accumulation.
Hope this helps somebody out there, good luck.
EDIT: By the by, thicker clothing obviously has the potential to accumulate more odour and if a change of clothes is not possible, wearing a thick sweater or sweatshirt while smoking ganja is not recommended if you want to keep a low profile.