For my first sub 3 and the two that followed it, did the following:
-get more sleep
-add mileage (in the form of an easy 4 every morning)
-focused on core work
-ran with guy who were faster
-did lots of strength training
I got to 2:52 of peak mileage of about 70 MPW. and I am sure I would have gone faster if I had of paced better as I hit the 20 mile mark in like 1:55 or so.
FWIW, my last marathon before I started trying was about 3:12, after I put in the work I went 2:57, 2:55 then 2:52. I am trying to go 3:10 in december of this year.