By request: a hair post. I must preface this with the disclaimer that I don't consider myself remotely skilled in the hair department and I've only had (greenish) blue hair for a couple of months now. Laziness and cheapness has driven me to doing much of my hair touch-ups at home recently. Over the weekend I cut my bangs and trimmed the back in an attempt to get closer to a bobbed hairstyle and for the past couple of months I've been touching up my color at home. So, while not an expert I can share my personal experiences.
Dyeing: After my first foray into blue from a salon, I bit the true bullet by dyeing my hair at home. To achieve vivid colors like blue or pink you must bleach your hair first. This is not only necessary if you have dark hair because in addition to lightening your hair the bleach makes your hair more porous so it can better absorb the bright colors. I bleached it using a Splat kit which comes complete with bleach and blue hair dye (this is the color you'll find in my Iceland posts); I merely followed the instructions on the box which were fairly straightforward. When I returned from Iceland I wanted my hair to be darker and bluer, so without additional bleaching I added a mix of Special Effects Blue Haired Freak and Manic Panic Atomic Turquoise. Basically I mixed about 50% of each color in a bowl and applied the mix all over to my dry hair, left in for 1 hour and then washed out with cold water. Although it's already faded some, this is the color I have right now; I really like how there are streaks of bright blue among the more green shades and how differently it looks depending on the light.
Maintaining: Because blue fades quickly you should try to avoid washing your hair as much as possible; dry shampoo helps in this process. I'm using Suave Keratin infusion for my days between washes. When you do wash your hair use cold water, as cold as you can physically handle. I've also been advised by more experienced hair color people to look for shampoos and conditioners that are sulfate free. Personally, I'm using Bumble & Bumble Color-Minded shampoo and conditioner. Blue will stain when wet for a few days after the initial dye, so I recommend using dark towels and if you shower at night avoid going to bed with a wet head. I haven't had any problems with my blue staining or running from rain or sweating, so I only worry about towels and sheets immediately after a shower.
Styling: Most days my hair is fairly straight and I recently bought a round brush to style it when it's still damp; a new brush since I threw away my old brush and hair ties when I went pixie a couple of years ago! As a perpetually straight-haired girl I've always been envious of people with curls, so as soon as my hair grew long enough I went back to my curling habits (as evidenced by many posts) and a number of people asked how I was styling the more curly looks. I use a small curling iron and basically make ringlets while holding the iron perpendicular to the ground. Once my hair is curly all over I break apart the ringlets with my fingers for a more undone look. Most days I prefer to not use any styling gel or hairspray so that the curls fall and loosen as the day goes on and my hair looks softer and more wavy towards the end of the day. It looks quite different with my new bangs, perhaps a little Clara Bow-esque..?