The free online tutorials are really really good. BUt, I learned more on buttercream after watching one DVD from Sugarshack than I had from all the youtube videos online. She does a great job on showing step by step how to get a smooth cake, and explains why these techniques work. It is very informative, and although it seems like a lot for a DVD (in comparison to any movie you buy in walmart) it really is a great deal, and investment into your hobby or business!!
I let it set up in the fridge then, dip my small spatula in just a little water, not a lot then lightly go over the icing with it or use a spray bottle you just want very little though too much will ruin your icing
Go to jessicakes blog, she has a great video on the upside down method. Its the best way to get nice sharp edges. Also she has a little story on the aussie method, I havent tried that one yet, but it looks genius.
An easy way to smooth icing is to let it set-up or harden a bit if you will. Take a piece of Viva paper towel or parchment paper and lay it on top of the cake gently rubbing. The heat from your hands will smooth out any imperfections. Be aware that if you rub to long in one spot it will melt the frosting and stick to the towel or parchment.
I really hope this helps. It works really well and is probably the best thing my wilton teacher taugh me other than how to get an even cake WITHOUT cutting it
Also i discovered another way. I take a styrofoam plate, cut it in half making sure the edges are cut straight and smooth. I used this to smooth out the icing on the cake, it is so easy and convenients