When I'm initially trying to make the switch I wait an extra week to make sure the snake is good and hungry. Gives me a better chance of him/her striking immediately. I try to feed as close to night time as possible, and I make sure to put the f/t under a heat lamp for about five minutes to warm it up to better attract the snake. Most hatchlings switch over for me after a week or two, adults can be much more stubborn. Generally I like to get everyone switched to f/t as quickly as possible since you can buy them in bulk so much cheaper than live. I also hate rats and don't want any live ones in my house!
Anyway, patience is key. There might be some skipped meals as the snake holds out for a live prey, but generally they'll cave and snatch up the f/t offered. I only have a few adults that force me to feed live. The rest I've trained to the point where I can put the dead rat on the floor by their hide opening before going to bed, and the next morning it's gone. if you're working with a hatchling you might not want to wait them out as long as an adult you're switching over.