I wear braids regularly as a protective style for 5-8 weeks at a time, leave my hair out for a few days or weeks and braid up for another 5-8 weeks. How I care for my braids largely depends on how long I have em in for.
I use braid spray on my roots, dividing my hair in sections and spray along the length of my hair. If you're not a fan, witch hazel is a gentle cleansing agent that will refresh your scalp just as well. Then i oil my roots, usually with grapeseed oil but lately it's coconut oil. i make sure to massage my roots. If I see a lot a fly away hair I may rebraid w/a moisturizer. Then I cover w/a braiding cap at night. Every 3 days I use a dry shampoo to cleanse my scalp as my routine's first step.
Once when I had my braids in for almost 3 months I used diluted shampoo & conditioner every 2 weeks for washing. To moisturize I did the same routine as above, but would fix almost every braid.
I always retain length this way, especially with the scalp massaging. About 3/4 inches every 6 weeks.