I knew early on last Sunday's marathon that I was feeling very good, and fortunately I was strong enough to maintain a (relatively) strong pace through the entire 26 miles.
Many factors go into a good marathon- long runs, tempo runs, intervals, diet, weather, hydration, etc. One often over-looked piece of the puzzle is a proper taper.
First of all, besides the obvious identify what are you trying to accomplish with your taper. You want to go into your marathon fully glycogen loaded, hydrated, and rested. You want an abundance of healthy, oxygen-rich red blood cells. You also do not want to taper down so much that your blood volume begins to drop or you begin to lose fitness.
I have no idea if what I did in the week prior to this marathon was optimal; all I know is it certainly did not hurt me and I felt great on race day.
Sun: 9.5 miles easy
Mon: 5 miles in 35 min (7 min pace)
Tues: 6 x 800s in 2:55, 2:53, 2:51, 2:54, 2:51, 2:49 with 2 min recovery
Wed: 5 miles at 7:20 pace feeling a little tired
Thurs: 3 miles in 18:44 pushing it pretty hard
Fri: 2 miles in 12:30 again pushing it
Sat: easy warm-up then 1 x 800 in 2:57 followed by 2 x 30 sec of hard running
See this post for some of the research behind this protocol.