- ONLY trim if your hair has split ends or the hemline starts to look too thin or uneven for your tastes
The whole "trim your hair every 6-8 weeks to make your hair grow longer" is just false. Your hair grows from the roots and your roots do not know what is happening to your ends. You only need to trim your ends regularly if you hair is split prone (fine or damaged hair can be like this).
If your hair has been chemically colored, permed, abused mechanically for years (long hair BSL+) then it has a tendency to break off giving you the appearance that it's not growing. Trimming off the damage regularly can help by getting rid of split ends. Split ends travel up the strand and entangle neighboring strands causing more and more damage, always trim off your split ends.
After a certain length (about mid back length) you have had those ends for several year (about 3) which means that all the combing, washing, wearing your hair down to be tangled by the wind, having hair rub against clothes, get caught between your back and chairs, getting caught under seat belts or under purse straps, etc has worn down your hair mechanically. This means that even if you don't color your hair chemically, or heat style it, or blow dry it, or perm it, your hair is still worn out after a few years from mechanical damage. This also means that the longer your hair is the more your ends are prone to breakage and split ends so you should be more watchful of split ends, breakage and do trims a bit more often than at shorter lengths.
You can prolong your hair's health by keeping your hair in buns or braids which will help avoid mechanical damage. You can also find more info on that in these entries: How to avoid unnecessary hair damage, Why stretch washes?, What you need to know about growing long healthy hair.
- ONLY trim as much as you need (ex. if your hemline looks thin for the last inch only trim that one inch, or if your split ends are only at the very ends trim 1/2 or 1/4 or 1/8 of inch however much you see necessary)
Hair grows about 1/2 an inch per month (on average) so trimming off 1/2 an inch every 8 weeks means that you will gain 3 inches of length per year in stead of the 6 inches you'd grow if you didn't trim at all that year.
- ALWAYS trim less first because you can always trim more but you can't untrim (is that a word?)
- if you see split ends through ought your length then trim them individually it's not worth loosing 10 inches of hair because you saw two split ends 10 inches from your hemline
- ALWAYS cut perpendicularly to the hair strand, imagine it as a T where the scissor is the top part of the T the horizontal line ("-") and the hair strand is the vertical line ("|")
- NEVER use a razor
Hemline (full) trims: http://feyeselftrim.livejournal.com/
S&D (search and destroy, or snipping individual split ends): https://sites.google.com/site/habioku/about-hair/self-trimming
If you want to self trim buy a professional hair scissor and ONLY use it to cut hair so it stays as sharp as possible.