Trust me, there are so many different tricks to get snakes to eat if they are being fussy... you can't actually run out of ideas. I've been keeping snakes for many years, and I know that even I haven't heard them all. There are also many that I won't quote to you because I find a lot of the tricks out there to be pretty disturbing.
I tend to stop short of too much trickery however, and try to get a good feel for the snake's situation:
If the snake's cage is in a high-traffic area (i.e. center of the living room) that could also make him not want to eat. He'll feel most secure if he is kept reasonably warm and dark and still. When he feels secure, he'll also be more likely to eat.
I like feeding snakes in enclosures other than what they live in - for my smaller BPs I use a tall 19qt rubbermaid container with a locking lid. For the shy ones, I cover it with a blanket and leave them completely alone for 20-40 minutes. The paper bag thing that someone else suggested would be similar, and somewhat cheaper - just watch your aim with the stapler!!! Of the hundreds of snakes that I have kept (no joke - I had a large collection at one time that I had to sell off), I have only had ONE snake refuse to eat in a separate container, and it's my youngest female BP that I have right now (grr).
Some snakes, especially BPs, just get hooked on mice, so it won't matter what you do with a f/t rat, it just won't be appealing. If you are trying to switch not only from live to f/t but ALSO from mice to rats, you can't really be sure which difference is causing your troubles. You could have a determined mouser.
A lot of people are convinced that mice are inferior food. Since I personally hate mice (I used to breed mice and rats for several years), I would tell you in jest that mice are inferior... but they are perfectly good snake food.
The only problem with BPs that are addicted to mice is that they end up needing more prey animals in a feeding to equate the meal we think they should get in a rat. It can get expensive if you are buying them from someone else, but hey, if your snake will still eat live mice, the most important thing to do is feed it what it will eat. So what if live mice are a pain in the rear? You want your snake to eat, right?? I'm pretty aggravated because my largest female is mouse-addicted, but I'll feed her the mice. I like it a lot better that she eats, and worry a lot less about WHAT she eats, so long as she DOES eat. Remember the priorities here: keep your snake happy and healthy and all will be well.
The flip side of all of this of course is that if this has been such an ongoing problem, and if I have misunderstood, and live mice are not still appealing to your snake, a visit to the vet may be in order... just to ensure that there are no internal parasites, unseen injuries, diseases, or otherwise unknown maladies which are putting your snake off his feed.
Don't give up. 
~Rebecca
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0.1 Dumeril's Boa '04 (Courtney)
1.2 Ball Pythons
[1.0 '05 Orange Hypo (Specter)]
[0.1 '05 Het Orange Hypo (Sylvia)]
[0.1 '03 Normal (Sue)]
0.2 American Pit Bull Terriers (40lb darling lap dogs:Brandy&Mara)