I am in no way an expert but will offer advice the best I can.
A sew-in, in my experience, can be an excellent way to acheive growth and retention as well as a major setback. What makes the difference is the way you take care of YOUR hair during your sew-in, as well as before and after.
If your hair isnt being taken care of prior to you getting a sew-in, the likelihood of those practices will continue when you do get your sew-in. So first things first,learn and practice healthy hair practices. To the OP, if you want to get into the pratice of getting a sew-in for growth, you should first ensure your moisture and protien are in balance. This will help tremendously with your breakage.
During the time you have a sew in, treat your hair the same. Shampoo and condition your as you would if it werent braided. The best way to do so is with diluted shampoo and conditioner. MOISTURIZE your hair!!! I personally put EVOO in a color applicator bottle and apply to my braid generously.
After you're done with your sew-in be CAREFUL during the take down. Have someone you trust or yourself cut only the thread. Its probably best to use a thread cutter, this way you can worry less about cutting your hair. NEVER NEVER NEVER wash your hair before detangling. Keep in mind you havent manipulated your hair in a while so baby it during this time. What has worked best for me is to saturate my hair in a conditioner or oil to detangle, something like a pre-poo. Once you have carefully detangle next you can proceed to shampoo and condition your hair as normal.
I hope this helps you to a more succesful sew-in as well as abundant hair growth and retention.