I think that the changes you personally choose also depend on factors like the instrument being played, the "user friendly" keys playable on that instrument, and ability of the player to play in the new keys. Vocal range has already been mentioned as a possible limiting factor on the key change you engineer.
One folk group that really impressed me with their creative key changes was the New Christy Minstrels. Here's a midi of 'Today', where they used two half-step key changes with a seventh chord to make the connection. In the first instance they made the change halfway through a chorus... which they don't even finish. But its effect is beautiful!
My favourite modern orchestra leader who performs magic with key changes is James Last. If you ever get a chance to hear his albums like Op Klompen (Dutch folk songs) or Last of Old England (English traditional tunes) you will find them to be textbooks of brilliant arranging.
I love to muck around with key changes when I sequence midi files for my tunebook website. (It's fun because I don't have to play the music... just flick my mouse!) You, too, can dabble if you have midi sequencing software.
Here are a few pleasantly successful experiments...
Ah! Si mon moine voulait danser!
La feuille ronde
Canadian Folk 'Overture' (Get a coffee before you click this one... it's five minutes long!)
- - -