That's a good question! I do this frequently to brush up on my skills. So what I do is find a tutorial on http://design.tutsplus.com, http://blog.spoongraphics.co.uk, http://vectips.com or http://colorburned.com/category/tutorials/ that's relevant to something I want to learn and I do as much of the tutorial as I can to pick up certain techniques I want to learn. Other times I make Pinterest boards of styles I like or look back at Pinterest boards I made in the past or one's I follow. I pick one that I really like and I copy it the best way I know how to just to get a feel for the style, spacing, colors, layout, etc. When I'm copying something, I notice certain little details that I didn't think of before. And then I just trash it, but I keep the skills I learned with me forever.