what is a fool proof way to spread frosting on a cake so that it is very smooth

http://cakecentral.com/articles/71/how-to-create-faux-fondant-the-paper-smoothing-towel-method-viva

http://cakecentral.com/articles/126/quick-easy-smooth-icing-using-a-roller-melvira-method

Both of these methods work really well. You have to have a crusting butter cream. There have been many threads on each of these methods. I use the Viva method most of the time and have great results! HTH

It helps if your icing is silky smooth to begin with. Make sure it's not too airy.

thanks for the tips anymore?

Don't over beat your icing. If you have too much air, it will be impossible to smooth. I like to smoosh mine on the sides of a bowl with a large spoon or spatula to get some of the air bubbles out.

You can also make sure that when you make your buttercream, that you make enough so that the beater is completely submerged under the icing. I usually have to make two batches. But this usually cuts down on the bubbles as well. To see an example of this, check out this video:

When you are icing the cake, put on twice as much as you think you'll need. Then use a bench scraper or a paint scraper from the hardware store and make one continuous swipe around the outside of the cake. A turn table is very useful for doing this. I consider it a necessity. When you have gotten it as smooth as possible then let it crust (if you use a crusting recipe) and go over it with a Viva paper towel or a piece of parchment or waxed paper.

I use both IMBC and crusting BC recipes, and the smoothing is a little different for each one.