I believe there are medical treatments that can help, but no creams or lotions seem to work. Time will fade them to a silvery color that is harder to notice. I have stretchmarks on my hips/breasts just from puberty and on my stomach/outside of my thighs from weight gain/loss.
I find that self tanner helps camoflage. Moisturize really well with a regular lotion for a few days so it doesn't go on streaky, and exfoliate before you apply.
Sadly, it's basically a tear in the skin, a scar. I think there are procedures (laser treatment, microdermabrasion) to get rid of them, but that would be way outside of my price range.
Mine were bright purple, but now they are white and shiny and not very noticeable. Keeping your skin well moisturised might help their appearance a little, but as Glory says, the skin is torn, and can't be mended. I personally wouldn't try any of the cosmetic procedures, once you start your basically committed to having top ups for life, as none of them offer a cure, just a temporary treatment, and it gets expensive. And no one knows the lifelong effects of having these treatments yet.
The ones I really hate are under my arms! Eek! Those ones still are a bit purply looking, but they're getting better as the weight goes down.