I was noticing the other day at bellydancing that even the really thin girls have stretch marks. I thought I got them from weight gain, but apparently a lot of girls get them just due to developing hips and such. Self tanners do help cover them, or maybe the tattoo thing QT is talking about.
As for the spider veins, I have some of those, too. I think some vitamin deficiencies can cause weak blood vessels which then rupture. Or wearing high heels a lot and putting strain on your legs can cause them. I'm pretty sure there's a procedure to get rid of them and varicose veins, but you have to go to a dr and it's probably expensive.
Tanning lotion helps cover mine up, too.