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FULLNESS FOR FINE HAIR - can be achieved by two dimensional coloring - have your hair colorist mix two or three shades of color and apply them via using a highlighting technique. If you do not like to sit for a long time then color your hair in one step single process. The color "swells" the hair up and creates a little of fulness.
You should keep your hair no longer then a shoulder length. The blunt cut gives an illusion of a wider hair at the bottom. But if you are good with brushes, you can ask your stylist to layer your hair and use all hairdressers tricks to make your hair very thick and full.
Your hairdresser may use texturizing scissors in about middle of a hair shaft. The shorter hair, if blow dried properly over the round brush, will "lift up" longer hair and therefor create again a fuller look.
Using a right shampoo for your hair type can help your hair look fuller. And remember - do not use too much of styling products, they will weight your hair down. For fine hair I love to use a small amount of mouse. It is very light. Some experts recommend use of herbal products instead of styling products. Apply it to the root area only, blow dry your hair away from the scalp, again adding body to your tresses.
Below are tips how to achieve fullness of fine/straight hair with styling.