Susan and I were discussing this just a bit ago...
The FEW times I have used F/T I would put them in a baggie and soak them in hot water to thaw them...the times were depending on the size...When I got them out they were usually not warm enough so I used a hair dryer and just heated them up a bit. You don't lay it directly on the prey item and it doesn't have to be HOT...just warm enough to get the bp's attention.
remember...what is warm to you may be too hot for your snake...so don't leave it under the dryer for long.
also...you may have a picky eater and might not be comfortable eating with you around...leave it in the enclosure overnight and see if it is gone in the morning.
just a side note....do not leave ANY LIVE prey item in the enclosure unattended.