Well, I find that simply going out in the sun a lot tends to lighten up my hair, but I have done the lemon juice thing when I was younger (you can put some in a spray bottle and spray it on your hair), and I've also used "Sun-In", which is a product that you can buy to spray on your hair and then spend some time in the sun so that it works.
I think you need a device that works like a tanning booth. It's UV rays that bleach hair. Ceiling lights won't work obviously. But if you can find a powerful UV light source and put your hair into it, it should work extremely well. Just don't burn your scalp, UV light causes skin cancer.
I have the same problem than you. Lemon juice is great but it can darken your skin so be careful when you put it in your hair. Chamomile is another great thing, I tend to have a spray bottle with concentrated chamomile in my fridge and just spray it in my hair before going out each day, it helps a bit and it smells great.
Beer is another awesome technique that I learned from a surfer dude. You put it in your hair and let it dry in the sun and it will help a lot, you'll have to wash it many times to get the smell out but I think it's one of the best ways.
Lemon juice is a good idea tip, also there is a product called "sun-in" I believe. It's just like the lemon juice idea, you spray it into your hair and then go out in the sun. I know people who said it reallyyyy helped to lighten their hair naturally.
Lemon juice works but like anything you try it works best when you let it dry in the sun. I've heard of chamomile tea; it's actually one of the main ingredients in my shampoo. I've also heard of honey, again another ingredient in my shampoo meant to lighten hair. The best thing is sun, as much as you can get!
swimming helps but not too much because your hair will turn green you can lighten your hair with tea any tea will do I don't recommend lemon juice I tried it it didn't turn pretty oh and I heard something about honey and mayo I'm not sure though hope this helps