1. Keep your hair up/off your shoulders often. let yourself wear your hair down on occassion but not too often. Lots of ladies here have show some great style options in their foki.
2. Low manipulation (find a good style that works on a weekly basis).
3. Balance moisture and protien correctly. This is really important!
4. Low/no heat to reduce any breakage. Look into rollersetting if you want to wear your hair out.
5. Enjoy life. Don't let being shoulder length drive you crazy and stress you out!
I didn't include stretching the relaxer because there's plenty of ladies here who are natural. Besides, I don't think thats in the top 5 things that you can do to make it past shoulder length. If a person can stretch successfully, then ok great! Thats all the better for the condition of their hair. But its not worth stretching if you get a bunch of breakage behind it.
I'm probably an inch to APL so can't say I'm there yet, but hair retention really did take off when I got past the tops of my shoulders.
-A