I like the idea of coder of the month which is already done in many online judges, but not in Codeforces.
So, I'm going to make a coders-of-the-month post by the end of every month, which will contain the top 5 coders of the month, not only top 1.
I'll calculate a score for every coder who participated in at least 1 rated contest during this month, this score will be simply the sum of his/her rating changes during this month, with the following rules:
1- The very first 5 rated contests for a coder will not affect his score.
2- A rating increase will be multiplied by a factor from 0.5 to 3.0, this factor will be higher if the coder's rating before the contest is higher.
3- A coder with score less than or equal to -600 for the previous 2 months combined together (without the 1st rule) will not be in the coders-of-the-month list for this month.
For sure the coders-of-the-month list will contain the top 5 coders with a positive score.
If you have a better idea to calculate this score or you have a modification for my idea, please post it here so we can discuss it.
Here is a example for coders of July 2012: http://codeforces.com/blog/entry/5170
And another example for coders of August 2012: http://codeforces.com/blog/entry/5183
And all coders of the month from February 2010 to June 2012 here: http://codeforces.com/blog/entry/5184 (the first few months are without the 1st rule)
Edit: Added shafaet_du's idea.