In 2000 ran Chicago in 4:34. Was pretty disappointed and knew I could do better...followed a pretty basic program - one long run of 20 miles.
Took 2001 off of marathoning and just ran...worked on getting my mileage up and buidling a base. No particular program - just running.
Tried Chicago again in 2002 and came up with a 3:29. Nice 65 minute PR, but still short of the BQ.
In 2003, hit Chicago for a final time. Ran 55 - 65 miles per week, 3 - 20 milers, hard 8-10 mile runs on Thursday nights and 30 secs BELOW marathon pace. Ran Chicago and popped a 3:07:08 to get my BQ. Did Boston last year and will be going back in two weeks.
The key for me was running about more miles and not being afraid to run hard. That same year I made my BQ, I did my half PR of 1:25:34 [Dexter / AA] and Crim 10 miler [63:15].
Just don't give up...get the miles in....run a decent amount of your miles at marathon pace and pray for good weather!
Good luck.