Do some research, get the facts, print them out and have all your replies to your friends' uninformed comments ready. Then sit down with them and ask them to have a civilized discussion regarding the effects of the drug. If you've done your homework, you should be able to easily contradict anything they tell you about it severely damaging your health or otherwise affecting your life negatively. Get them to understand that the only bad things about smoking pot are because it's illegal. Tell your friend who thinks your life will be over at 30 when you fail a drug test that (a) drug tests are extremely easy to cheat on or sober up for, and (b) not getting one job isn't going to ruin your life since there are always other jobs out there and some of them are for people who respect others' rights to do what they please in their free time.

Tell them it doesn't really destroy brain cells, that it doesn't give anyone cancer, that pot smokers who don't do other drugs have the same mental capabilities as non-smokers. And have the research to back it up.

http://www.drugtext.org/sub/marmyt1.html
http://www.drugpolicy.org/marijuana/factsmyths/