From a medical perspective, as far as I know, the only thing you can do is cut down/stop smoking.
Doctors (in the UK at least) calculate your risk from smoking cigarette in pack-years:
1 pack-year is 1 pack of 20 cigarettes a day for a year, 30 pack-years is 1 pack a day for 30 years
or 2 packs a day for 15 years
or half a pack a day for 60 years..
Obviously it gets confusing when you start thinking about rolling tobacco/spliffs/whatever but the general principle behind it is that smoking a little for a long time causes the similar harm as smoking a lot for a shorter while. The damage is cumulative. However this probably isn't hard-and-fast - smoking a HUGE amount for a very short time might cause disproportionate damage as your lungs don't have a chance to even begin to repair themselves. Also, everyone is different. For some people smoking one cigarette a day, or a few at weekends, is enough to do a lot of damage where as others smoke more but don't seem to get the same amount of damage.
I feel your pain about not wanting to cut down (I smoke cigarettes and used to smoke a lot of spliffs, and I find cutting down really hard despite knowing about the damage I am doing to myself) but it is the only way really. Be careful of supplements/herbal remedies - a lot are complete rubbish - check for
proper scientific research on them if you want evidence, and don't use them as a substitute for quitting/seeing a doctor if you are concerned about your lungs
Also, it depends what damage you are talking about. Tobacco cause various types -
1. Emphysema, which is damaging the tiny sacs (alveoli) in your lungs where O2 is taken in and CO2 removed from the blood, and is permanent
2. Bronchitis, which is inflammation of the airways, causing increased phlegm production ("smokers cough") and narrowing of the airways - it is reversible but can progress to irreversible airway obstruction (COPD or chronic obsrtuctive pulmonary disease is a term used for the combination of emphysema and irreversible airway obstruction often found in long term smokers)
3. Smoking paralyses the cilia lining your airways - little hairs on the cells in your airways which beat upwards, carrying all the crap you inhale up and out of your lungs. This is temporary, but the damage the crap sitting in your lungs does may not be.
4. Lung cancer - huge topic, may be protective things you can do but it is debatable, also caused by things other than smoking, and there are several different types - but is predominantly seen in smokers
5. Damage to the heart, blood vessels, brain etc - but you are asking about lungs so won't go into this!
Essentially, as far as I know, the only thing you can do is limit or stop smoking, and the sooner you do it the better.
Quick guide do damage caused by smoking, but there are loads of sites about this. There are loads of medical texts and research papers too going into it in a lot more detail but I haven't got time to do a search right now...
Your lungs can repair themselves from a certain amount of damage but things like emphysema are permanent. Think of it as how the liver can repair the damage done by drinking if you've only got to the fatty or enlaged liver stage, but once you get to cirrhosis it is irrepairable - but the lungs aren't as good at it as the liver..
Sorry for the information-overload! Hope it makes sense and is helpful

and good luck!
Despite writing a mini essay I'm going to move this over to Healthy Living as I think you'll get a better response from the posters in there, but pm me if you're not happy about this decision..
BDD > HL