After smoothing the cake (Viva method) could I use a papertowel that has a pattern? I am thinking a diamond pattern or flower pattern to make an impression on the sides of the cake. I think there are paper towels that have raised designs that would work. Anyone tried this? Is it difficult to match up?

I have done this before, and it can be difficult to match up. Also, it doesn't give a very strong impression.

What can I use to get a strong impression?

There are a number of different companies that make impression mats for cakes. But there are any number of things you can use from around the house too.
Textured Shelf Liner
The stretchy bags that Lemons/Limes/Oranges come in
Flourescent light covers (they come in large sheets)
Heavily textured fabric/vinyl
Impression sheets for artist
Rubber Stamps

Hope that helps some.

I have used rubber stamps from Michaels or AC Moore's and they also have these styrofoam thing that I found in the kids isle of the store and they have flowrers, animals, words etc.. that you can put an impression in something to. I have not tried with Butter cream but I think that if it is set that it should work, it is great for fondant too. HTH

Thanks so much for the ideas. I am going to look around and see what I can find. You all have been very helpful!