I use a nifty little tool from http://sciral.com/consistency. It is perfect for tasks that are hard to fit into a conventional calender like:
schedule an appointment with my dentist somewhere between five to seven months from now
Call mom not later than ten days after our last conversation
Go to the gym at least once a week, but no sooner than two days after the last training session
You also can use the tool to easily track how sucessfully you are in implementing a new habit.
The good things: the program is cheap, easy to use and available for windows, mac and iOS devices
The downside: data are not cloud aware, but stored in local text files, so it is hard to keep them in sync across several devices. The new SkyDrive might be a solution for this - still testing.
There has been no significant development of the program for about five years now (except for bug fixes and porting to iOS)
For me it still is quite useful - there is an evaluation version available, if that sounds interesting to you.
An optimist is someone who fills in a crossword puzzle with a ballpoint pen - Karl Farkas