I'm a stylist and the lemon juice/sunshine may work, but it can turn into a streaky, brassy, spotty mess (think of the sun-in calico cat look)... especially if it's going to be applied to roots only, as a touch up. If your hair is dark, it will give your hair a more reddish/auburn hue, it only lightens so much and not enough to go blonde, unless your hair is fairly blond naturally to begin with. Tips on using lemon and sun if you want to try it:
http://www.hairstyle.com/hair-advice/lighten-hair-with-le...
Bleaching with lemon juice will also dry your hair out horribly, and cause breaking from brittle hair, so you would need to do some heavy moisturizing treatmenst before/after.
Henna is natural, but it's too dark for what you are trying to achieve and can also be damaging to the hair, and you can't color over Henna, you would have to cut it out if you ever wanted a color change.
If you get fine highlights, use a low volume developer (30 vol) and only go no more than 3 shades lighter than your natural color, and use a toner to achieve the final color without blasting your hair under lights, then there shouldn't be a worry to using salon lighteners.
For a natural way to break up the color line between your roots/hair growth and blonde color, would be to do a mix lo-light and hi-light so that line fades out and you won't have to color your hair as often throughout the year.
The thing is, there are more 'healthy' options for deposit only (going darker) colors than there are for lighteners for specific reasons.
Here are some natural ways to keep hair healthy.
http://www.womenshealthcaretopics.com/bn_hair_Highlights.htm
Here is an eco friendly color, but I believe it is available to professionals only:
http://www.ecocolors.net/
Here is another good one:
http://www.actnaturals.com/
http://www.natural-living-for-women.com/natural-hair-colo...
(or what Andrea said, how could I forget about Aveda?!)
