What is an appropriate tip for a friend that does nails for a living?

I'm just going to get them painted, no tips or anything like that... but she's a nice girl and in alot of classes with me at school, so I don't want to jip her... but I dont want to give too much because maybe I wont always be able to give her a huge tip, u know? so what's appropriate? thanksok so if she is only charging me $10 what should I tip? 15 percent would be only $1.50...But I think $5 is too much! how about $3? or does that make me look cheap?

Always tip at least 15 percent. This is her livelihood remember.