Don't be tempted to go too dark - even just the slightest tint will make a HUGE difference if you're naturally quite fair! My sister did hers a few years back, and, being a bit stubborn/dim/impatient!! asked for darker than the beautician recommended. Oh dear - how we laughed!! (Aren't sisters cruel?!) But she looked so hugely different, and it just didn't ever look natural. She had to wait a good few weeks for it to fade, and couldn't really disguise it in the meantime, bless her.
She has it done regularly these days, and now that she knows to trust the experts, it looks fantastic!! (And she's forgiven us now too.) It's definitely something I would recommend.