1 - Plan ahead:
Hair dye can be regarded as the point-of-no-return. Even if you can dye it back to your natural color, it won't feel the same. If you're trying to lighten your hair a whole bunch, consider the fact that your hair will become much dryer and coarser than you're used to. Be willing to change your haircare routine for the very best care and results for your hair.
2 - Seek professional assistance hair coloring:
For the very best results, get your hair professionally dyed by your favorite stylist or a professional colorist. Most stylists love having some funky, challenging style to do and will love to create it with your hair!
3 - Use a flat iron to seal your hair color:
Once the hair is freshly dyed, make sure you/your stylist goes over it with a flat iron after it's dried. This helps lock the color into the hair shaft.
4 - Wait before washing your hair:
Don't wash your hair for at least 48 hours. This is a mandatory step for anyone wishing their hair-color to last more than a week. Ideally, wait as long as you can. If you're only dying a small section of your hair (bangs, streaks on top, tips etc) avoid hitting this with your shampoo as much as possible.
5 - Use mild, color-treated shampoo and conditioner:
Most shampoos bottles will state if they are designed for color treated hair.
6 - Use a moisture-rich conditioner:
Dyed hair tends to be dry and straw-like. Don't hate your hair for it--nurture it.
7 - Wash your hair in the coldest water you can stand:
Not just the first time, but all the time. Hot water dries out hair and skin, leaving it dull. Cold water will not only help you keep your color but also help you keep your health and shine.
8 - What to avoid:
Avoid harsh soaps, chemicals (especially chlorine), and any shampoo that specializes in "clean" hair. These are also dye-sucking vampires.
If your hair is particularly vibrant and unnatural (bright reds, purples, blues, greens, pinks, etc) try and keep out of extended periods of exposure to sunlight and water. Sunlight has been known to work somewhat as a natural bleach, so be careful if you have any long beach trips or life guarding stints planned.
9 - Redye often, but not too often:
Your hair shaft is delicate, and should only be dyed once a month or so. Any sooner than that and it will be prone to breaking, splitting, tangles and straw-like texture.
