1. Reds wash out EXTREMELY QUICKLY.
Not kidding. Be ready to pay big money on your fresh new red hair if you hate how the color fades. I would suggest burgundy or more purple-y reds that fade to a nice color.
2. Box dyes are fine, but the salon is worth it.
Of course, it definitely depends on your salon. I currently visit Studio 21 and they're prices are relatively good and they do an AMAZING job. There are always the exceptions where you go to a cheap salon and come out in tears. Just know you get what you are willing to pay for.
3. After permanently dying long hair several times it will dry out like a desert.
I've been dying my hair a purple-red for a little over a year and it has wreaked havoc on my ends. They are extremely dry and mat easily; I have to deep condition every time I wash it. On the plus side, it doesn't get as greasy and nasty as it used to.
4. Don't be afraid to try new colors, but also know certain ones are hard to change back; and even harder to go back to your natural hair color.
Before I dyed my hair they told me if i wanted to go back to my natural color it would take about a year of stripping my hair. Luckily for me, i fell in love with my new color-- but sometimes it doesn't work that way for everyone.
5. BUY GOOD SULFATE FREE SHAMPOO. THIS IS NOT OPTIONAL.
This does not mean buy the most expensive sulfate free shampoo. It just means you're going to have to do a little research. Depending on hair color, there are some good brands that are made specifically for that one color. I would suggest Redkin Color Magnetics for red/purple hair like mine.
But no matter what you do, love yourself and your beautiful flowing locks of hair and stay fabulous.