No secrets, just lots of running. So, run 3-4 or preferably 4-5 times a week, start increasing the length of your long runs (gradually of course) so that you're used to running at least the starting mileage for your schedule, and preferably more. Join a club (if you haven't already) and run their cross-country season races during the winter, and/or run a parkrun every Saturday morning.
Oh, and if you're following a 16-week schedule, start it at least 18 weeks before your marathon. That way when you have a week or two off due to a cold or a niggle, you're not falling behind on training. You can always repeat a couple of the later weeks (and be better trained) if you -don't- need the time off.
HADD training might be useful for base training.