You could chill the cake tiers until the bc firms up a bit, that helps.
I don't know if you refrigerate your cakes, but I always do (fondant or not) and if I'm adding a tier onto another and it's buttercream, I have to make sure the buttercream is really firm before I attempt to transfer. I then also use a viva towel just to do some final touch ups and always have a border! Good luck!
Sorry to hear your having problems with stacking but usually wach layer is iced in dividually and put in frig until ready to stack, this keeps it cold and helps alot. here is a you tube video to kinda help out and there is many more if you need to do some searching for what will help you best. Good Luck
http://www.youtube.com/watch?v=u-_PauOVx74&feature=fvsr
I have had problems when I need to stack my tiers at the reception site. They can't be chilled then. I wish I had a simple solution. I do my best and fix any messes!
How many teirs are we talking about? You should be able to stack and transport 3 teirs and if you have 4 than do 2 and 2 or 3 and 1, you should have something done before getting to the venue, yes? 
I wrote this in another thread, when someone asked "Where should my hands be when I'm stacking a cake?":
One hand on the spatula and one hand underneath the cake (on its own cake circle) supporting the weight.
If you're using SPS, you can lower one side of the tier onto the plate, remove your hand, slide the tier into place across the plate with your spatula, lower the cake the rest of the way, and remove your spatula.
If you aren't using SPS, it helps with positioning if you're placing your tier onto another cake circle (just as you would with a single plate separator). So rather than trying to lower it on top of a buttercream surface (while denting the lower cake, trying to get your hand and spatula out without making a mess, and possibly disturbing the dowels), you're lowering it onto a corrugated cake circle instead that's already stuck in place on the surface of the lower tier. Doubling up on circles is easier than risking damage and causing internal support issues, and you'll still be able to hammer a center dowel through all tiers without a problem.
Here's the entire thread:
http://cakecentral.com/cake-decorating-ftopict-710889-spatula.html
you would cover them with plastic wrap, right?
No... not necessary, cake circles are made for this purpose.
When you put them together, would they slide, or would the icing hold them together?
A dab of icing holds them together. (A center dowel does too.)
Here's my version of step-by-step pics that might give you some ideas:
http://cateritsimple.blogspot.com/2011/01/how-to-assemble-stacked-wedding-cake.html
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