I carry 3 putter molds when I'm being serious:
1 D Magnet
1 D Challenger
1 D Zone
The Magnet is my primary putter, but if I miss an easy putt (within 30') then I'll throw my Challenger on the next hole, then back,etc. This is working really well for me. It's something mental, and when I miss a putt I lose confidence in the mold, so throwing a different putter in the same mold just doesn't seem to work. Changing to a different one has yielded some great results. Blaming the disc, and not me (I know .. I know) seems to keep my confidence level up. It's easier for me to say, "The magnet is not working for me today" than to think, "God, my putting is AWFUL right now."
I carry the Zone for upshots, short drives (I have an ESP Zone for longer short drives), and stiff headwinds. Depending on how the day is going, I've even switched it in for my primary putter (because it feels better, it's high-tide, or it's the 3rd tuesday of an even-numbered month in a leap-year). It's what ever I mentally feel like I can hit the putt with when I walk up to my lie.