Early in the game, I pic up everything that isn't nailed down, so I get overencombered and have to sort through it all. Now after I get my first house, usually breezehome, I get a little bit more picky and I run "home" more often. Books, ingredients, dragon bones/scales are never tos'd, but I have dropped a higher value item just to keep a book, because I didn't have it.
I tend to sell by weight first, that way when the vendor runs outa money, I'm not leaveing with as much weight. I have bowls full of gem's in my Markarth House, they glisten soooo pretty in the candle light...
