Firstly, stop getting it cut short!
It seems so obvious, yet, for one reason and another, we can find ourselves walking out of the hairdressers with the same length hair we had a few months back.
Hairdressers often have different ideas about hair to ordinary people. If the condition and cut isn't perfect, they tend to want to get snipping. This is the time to be very clear about your objective to grow your hair long and stick to that. Be firm! "Just the ends" really means "Just the ends".
Don't let a hairdresser tell you that you need to lose several inches. The chances are you don't - not if your plan is length and you aren't looking for perfect even lengths.
If you are really serious about growing your hair out, just get a minor end-trim once or twice a year.
Secondly, condition it well
To get your hair to grow out without uneven lengths due to breakage, you need to be serious about conditoning. You should be conditioning every time you wash your hair.
Thirdly, don't break it
Be careful when you brush. It's easy to break a lot of hairs if you are rough with a hairbrush.
Don't brush wet hair. Wet hair is stretchy, but it can easily stretch beyond it's elastic limit and become weak and prone to breaking.
Keep your hair brushed. Tie it back when out in windy or damp weather so it doesn't get knotted up and caught on things.
Plaiting your hair loosely before going to bed can be a good way to prevent a build up of knots when you are asleep.
Fourthly, enjoy your hair!
Growing your hair out will force you to become a little creative if you are to find ways to make it look good through the growth process. Clips, combs, ties, slides are all good ways to arrange your hair to survive each phase of growth.
It isn't feasible to assume you are just going to let it do it's own thing for a couple of years, in the hope you will survive til it reaches the length you are looking for. You will very likely get depressed and have it cut. Avoid this phase by finding ways to work with each phase of the growth.