Here is how I go about switching my balls:
1. Feed live of the tongs - get them used to seeing the tongs and associating them with feeding time, even if it's just to hold the live guy down till the snake takes it (I can feel PETA coming after me now).
2. switch to fresh prekilled off the tongs - if you've got a snake with a good feeding instinct it shouldn't be too much of a jump to taking prekilled food, twitch it around a bit.
3. then on to f/t off the tongs - I heat mine up before serving (98degrees) most of them are pretty conditioned to taking the p/k by the time I get to this step and usually go right after a heated f/t.
And there are some that just wont convert and some that go back and forth. It just happens that way. Good luck!!!
Hope this helps.
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Tosha 
"Stupid is forever, ignorance can be fixed." Don Wood
"Of course, that's only my opinion...and I believe I am God." =) Chris Bianco
7.33.0 Ball Python (Harry and Fluffy and gang)
1.0.0 Angolan Python (Skywalker)
0.0.1 Green Tree Python
0.2.0 Feline (Pippen and Pandora)
0.0.1 Dessert Tortoise (Pope John Paul)
2.2.1 Fish (1,2,3,4)
0.0.0 Pool skimmer rescues for this season