Some great suggestions, I would also advise collecting examples of the best graphic design. (Scrapbook, Screen shots).
But more than simply collecting them try and analyze them to figure out why they work, what is it about the layout, fonts, colour, images that make them stand out.
If you flicking through a magazine look for ads make you stop for a split second longer than others and try and work out why.
I use my phone and take photographs of books, billboards, posters etc anything that I find striking.
Have a look at these ads both from the 1960s VW changed the game with their ads. They began using clever messages, huge empty space, less text, simple layout and images, clean modern fonts.
If you glanced at these ads one will stick in your mind and one will not. For me the key to good visual design.
I’d also have a look at Ted Talks on visual design, there are some great one’s up there.