Lisa: Dammit, mine's only shoulder-length.
It takes patience and discipline to get mermaid hair, as most people are not born with it. One cannot use hair dyes, perms, other chemicals, or heat. Mermaid hair cannot be damaged. Vitamins and minerals may be taken to nourish new growth and coconut or argan oils may be used as humectants to help moisturize hair. Eat lots of protein, drink lots of water, and take a multivitamin. You may even want to consider keeping a shoe box filled with vitamins and minerals.
Trim dead/split ends so you don't end up with splitting, unhealthy hair strands. It's better to have healthy hair/ends than length any day, even for mermaid hair.
If you have hormonal imbalances or any form of alopecia (hormonal or hereditary,) it will be harder for you to obtain mermaid hair. Over-the-counter products like Rogaine can help women who've lost hair due to hormonal issues, but do not expect to get mermaid hair right away. Also, vitamin D and iron deficiencies can cause hair loss, so you may want to get blood tests done if you think supplements may help you get mermaid hair.
Do not use hair brushes. Instead, use wide-toothed combs or fro combs.