1. Don't Expect to Have The Perfect Blonde After The First Dye
Especially if you have dark hair. It can take several attempts to lighten the hair fully and it may take place after several weeks/month so not to damage the hair too much. I'm lucky and my hair lightens pretty easy but every hair type is different and you can never tell how the bleach will work till its completed. Don't go in with high expectations and know that this is a process.
2. Go To A Professional
Hairdressers despise box dyes. I am partial to one every now and again when darkening my hair but never ever going lighter. Always seek a hairdresser you trust. Look at reviews or maybe you know someone who's gone to that salon before. It will cost a bit more but its worth it. Often if you do it at home, it ends up need correcting in a salon anyway.
3. Protect Your Hair With The Right Products
Bleaching your hair takes its toll on your hairs condition so make sure you buy products to keep it healthy. I use TIGI Bed Head Urban Resurrection Shampoo and Conditioner as well as RedKen Extreme Anti Snap and always use some form of heat protection. I also use coconut oil as a mask and try to give myself days where I don't wash it so to get natural oils back in it. Drink lots of water and eat the right foods to make sure your hair is getting the right nutrients.
4. Purple Shampoo is A Gift From God
Any purple shampoo will do, doesn't have to be pricey. The purple helps rid of any brassy tones that can appear and keeps your blonde looking blonde.
5. Hair Does Grow Back But Really Slowly
I once dyed my hair blonde- brunette- blonde- brunette-blonde within a year and all my hair snapped off. On the third attempt I was told my hair was too damaged but I was adamant I wanted it dyed back and then it all fell out and I felt like I had lost all my confidence. Please look after your hair and if its not in fit condition to be dyed please don't do it because it grows back but really fucking slowly.