I guess it could be summed up in one word: time.
I know of most of the tactics used in civ (even if I dont use them all) but now I cant keep myself up to date because applying each tactic requires time, and my games are longer and longer. I am lucky enough for the new good tactics to be referenced in the GOTM forum cause I have not visited the other sub-forums for months now.
I have improved constantly from last June to January 2003, and that was obvious in everything and can be seen in my GOTM score. The peak was my 5th ? place at GOTM 15 Russians. But that game ate my Winter vacation and New Years Eve. I knew I could never get back there due to my limited time. By that time I gave up playing other games but the GOTM. I usually played 4 games per month, all on large maps, but now its GOTM only and even if I finish around 20th, I dont have time for a new game. In GOTM 16, 17, 18 and 19 I kept decreasing the amount of time I played only to see that my final position in GOTM was not much affected (discouraging milking also helped).
Its interesting that on the learning curve I grew for 4 months (August to November), than I lost two games GOTM 13 and GOTM 14 only because I was pushing too hard like Darkness wants to

, and I got my best game in GOTM 15. From then to now I tried an optimization of focus vs. result so that must seem like a decrease in performance. I intend to stay like that, Im happy where I am and dont want more.
Im a little disturbed by the new players that pass me like a rocket and end up on the top 10 but with time I will be able to live with that.