I have one simple rule that I (magically) am able to stick with for so long - make sure you clean up at the end of the day. No file on Desktop/Downloads, file things in their rightful place - or delete right away if it's not important.

Sound mundane, but once you just did it, it usually took about what, 5 minutes? But the impact the next time you turn on your computer and see the clean workspace without guilt is great.

Another tip is the same as commenter about trash bin above - let software automatically clean things for you at certain time, and you'll never worried about it again.