Lemon Juice. But lots of it, it works just the same as "Sun In". Put it in your hair and then sit in the sun, works better - used to do this in when I was younger so I know it works!
Don't use lemon juice! It's really drying for your hair, the best stuiff is the john frieda sun streaks gel (no i don't work for them), it's not as drying and sun in turns your hair a funny colour. Failing that go to the hairdressers!
Just a word of advice - many hairdressers will not touch your hair if you've used Sun-In because it's so unpredictable and can seriously weaken it. Don't forget that any bleaching is permanent. The old fashioned way is lemon juice and that's also bad for you (sorry)
Oh and I forgot, I've used sun - in a couple years back and my hair got really fried so becareful when you use it or atleast go to a hairdress if you want to lighten your hair.