Here's how I do gradual tempo changes.

I know where I want the change to be.... start...in process...and at the new tempo.

Most of the changes happen over no more than 2 bars. Each bar has 4 beats.

SO..... I go in and ADD 2 measures where I want the change. I then go in and change the two measures and the 2 new ones to 2 beats. Song tempo might be 120. So the first measure is 110, the next 100, the next 90, and the final one is 80, or a hold.... depending if I want to stop of keep going. Normally I'm using it to stop so the last one is a hold.

You can speed up by doing it backwards of the process above.

This song has the result at the ending: http://soundclick.com/share.cfm?id=13017714

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.