I'm a hairdresser and I had a short Bob 2 years ago, my hair is now nipple length. I only started focusing on growing it out in March of this year (when it was mid shoulder length) so a good 4-5in longer than 10 months ago.
To start go to a hairdresser and tell them to take off all of the dead bits. After that a trim of just the dry/dead/bitty ends every 6-8 weeks.
I only wash my hair a maximum of twice a week. I really do try and go for once but if I have places to go then I will wash it twice.
Minimal heat styling. I mean minimal as in I never ever blow dry and only use straightening or curling tools about twice a year.
Never brush when wet.
Use hair protecting styles to sleep in - Learn how to style overnight (plaits, rollers etc.)
Oils on the tips of your hair each morning.
A decent massage and scrub when you do wash your hair as it stimulates blood flow which of course stimulates growth and hair renewal. Also brilliant for the scalp to keep it in good condition.
Do a double shampoo and leave conditioner in for as long as you can to give your hair plenty of time to soak up the moisture. Leave in conditioners are even better.
The aim is to keep the hair as strong as possible which means keeping it moisturised, away from heat, not handling it when wet and regular one length trims. Feathering is also very good but takes a very long time so not many hairdressers want to do it.