I use my regular all crisco buttercream for them. I know there use to be a recipe floating around out there that said to use it with them that was half and half, but I never saw the point of making a separate buttercream just to make them.
I use a soft medium icing as well. As you pipe you want the icing to come out and around the tip, rather than piping line exact lines.
I outline using a tip 2 or 3 to outline and fill in with anything from a 5-10. I don't do the extra layer of icing on the back, I just build mine a little thicker.
Great. Thank you for the info. Is there a point of making an extra layer of icing on the back to match the cake or is it simply to make the transfer thicker? Do they come out looking smooth or do you see where the different lines of icing all comes together to fill in (hope that makes sense)?
As I am piping lines I try to make sure everything is the same height, and I can always add a little in areas that might not be. After it is done, then I will lay a piece of wax paper over the top of it and gently smooth the back of it.
Since I've never done the extra layer I can only guess what it is used for. My guess would be to make it thicker/even. I can't see why it would matter if it matched the color of the cake or not.
TexasSuger: I did my first buttercream transfer today and it looked great! Thanks for all your hints. One question how do you avoid seeing the lines from when you are filling in? It showed up in one part of my transfer and not in another, perhaps my icing wasn't thin enough in the one area?
To avoid the wormy look, try not to pipe in lines, but instead let the icing build out and away from your tip as you move it.
Also I will toss it in the freezer for a short bit, pull it out and flip it over and run my fingers over the transfer to kinda smooth out any lines. And once on the cake you can always viva it. 
Great! I am going to try another transfer for this weekend. Do you know how far in advance you can make/freeze the buttercream transfers? I want to make it ahead of time but don't want it to get frosty or anything. Any advice?
I have successfully frozen them for up to a month. I leave them on the wax paper and either lightly wrap with Saran Wrap or pop it into a Ziplock bag if it's small enough. Then I keep it stored on flat surface like a cookie sheet, and place it in a part of the freezer where it won't be disturbed.
As I am piping lines I try to make sure everything is the same height, and I can always add a little in areas that might not be. After it is done, then I will lay a piece of wax paper over the top of it and gently smooth the back of it.
That's a great idea--I'm going to try it!
I think TexasSugar and I have the same technique, but I'll try to describe it in a different way:
You should keep your tip buried in the icing as you move the icing around. It should appear to be a flattened mound of icing as you spread it around--don't try to pipe in lines!
Sharon Zambito's sheet cake dvd has an excellent section on how to do FBCT. I'll try to find her preview.
Here's the link. The part you want to see is about 2/3 into the video where she works on the puppy. It's just a snippet, but it will give you an idea of how she does it.
All her dvd's are top-rate, and this one is no exception--I highly recommend it. There are also lots of other FBCT videos on youtube that might help you.
I've also used the viva right after the transfer has been put on the cake. It will actually left off some of the moisture and dark colors, which will help keep it from bleeding any, and I haven't had one stick.
Nah, you just want to do it quickly. After it gets softer and before it crusts it may stick, but they have always been pretty solid for me when I take them out. I kinda like seeing the picture on the viva after ward.
Do you use a butter or shortening recipe? I use shortening so I wonder if that makes a difference?
I have used both successfully. Butter gets really hard when frozen, but I have not had any problem with fragile fbct made from shortening-based icing.
I use all shortening, but that's because I use in my regular icing.
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