How do stretch marks appear?
Stretch marks are the result of extreme and sudden expansion or contraction of the skin. There is a layer beneath our skin called the epidermis, when your muscles grow, naturally you skin will have to expand too, and boy if the skin is not elastic enough that area will tear up, resulting in stretch marks. Don't understand? See, take a rubber band and stretch it as hard and as wide as you can. What do you think will eventually happen? The band will break, the same case is with our skin. Unfortunately there has nothing been discovered up 'til now that will make stretch marks disappear completely. They can only diminish with time (maybe years). They might become lighter, so that you can see them only if you look at them carefully.
Fast growing and non elastic skin are found to be the major reasons for the appearance of stretch marks.
Stretch marks are a good sign?? Are you crazy Rockie???
I think so, but no relation with the above. Appearance of stretch marks on your skin means that your muscles are growing at blazing speed, so much that even your skin cannot cope with the pace. That's one thing to cheer you up.
But does this give you a reason to like stretch marks?-NO WAY. Okay just imagine a guy with 18 inch biceps and a huge chest, but full of stretch marks, do you think the gals will crowd around him. I don't think so, at all.
Okay what can I do to prevent stretch marks?
There's nothing much you can do about the stretch marks you already have; as I told you earlier there are two main reasons for appearing of stretch marks, lets take a look at both and see how we can go to the root of both the reasons to eliminate the marks up to a good extent.
1) Improper nutrition- the one thing I have seen in most bodybuilders is that they gulp down grams and grams of protein and hate anything other than that, for you to be healthy you need to 
have a balanced diet. Enrich yourself with vitamins and minerals, Vitamin E is the stuff you should especially crave for, it makes the skin healthy and more elastic and makes your stretch marks heal faster. Vitamin E also prevents further stretch marks.
Take multivitamin and multi-mineral pills every day in limited quantity, apart from these you should also take antioxidant tablets, but strictly under medical supervision.
2) Growing too fast- The other reason for appearance of stretch marks is because of your muscles growing too fast, so what do we do about that, my advice will be to grow at moderate speed, so that your skin doesn't get stretched indefinitely all of a sudden. I did not heed to this advice earlier so as a result I had stretch marks (Thank God now they are diminishing now!) Let your muscles grow at just the right speed, if you have more than a few stretch marks, don't take any supplements, they will make you grow fast and eventually you will have more and more stretch marks. Oh yeah and one more thing, don't wear polyester clothes, they are found to have a direct connection with the appearance of stretch marks.
Doc's Advice -STAY OFF BODYBUILDING FOR SOME MONTHS!!
Around 5 months ago, I was just having the same problem as all of you. Those ugly lines did much more than just troubling me, I couldn't even show off my then growing (then rather small) muscles! Eventually I went to the doctor. He looked at my marks and prescribed some medicines for me, but then he said something that couldn't be any worse, the doc told me to stay from bodybuilding for at least 3 months. I was skeptical about his advice, I was afraid that those stretch marks won't go and add to that I would also lose my hard earned muscles . After hearing his advice though, rather reluctantly I agreed.
But see after those three months under his medical supervision, a lot of my stretch marks almost healed and are faintly visible, I m really thankful for his advice, see this is what he said. Please stay off bodybuilding for some months when you have stretch marks, I know you don't wanna give it up for even a week and I know you will lose some of your muscle, but you don't want a big body filled with big stretch marks, do you? I caught something sensible in this point, and didn't lift for about 3 months, I had to comeback hard in my training after it, but you know that was all worth it.
Finally guys before I end this article I want to tell you that if you have stretch marks, don't feel disappointed or sad, if you are working hard to get the physique of your dreams, you'll always have to face more than a few problems, stretch marks are only one of them. So, cheer up!
Remember this for every field in life -NO PAIN , NO GAIN.