Excess possessions not only clutter up our physical living space, but they create mental clutter, as well. Every time you see something that needs to be put away, or repaired, or completed, it either gets put on your mental to-do list or creates negative feelings if you can't actually take care of it anytime soon (worse if it's something you realistically can never do-- think of a broken plastic part you may be holding on to that just isn't going to work right, even if you do actually get around to trying to glue it).
My advice? If you can do it immediately, DO IT. Don't let that mental clutter accumulate. Take the extra minute to put away this thing or that. Time yourself. You'll be surprised how quickly you can take care of the little detritus.
If you can't take care of it immediately, but it's still important to do at some point, put it out of sight. If you can't afford to forget about it for a while, then- back to the beginning- do it as immediately as possible. You don't want this stuff staring you in the face, stressing you out.
If you come to the conclusion that you will realistically never get it done, or that it's not important enough to spend your time on, get rid of it. Don't feel bad. You weren't going to get it done one way or another, so you may as well not get it done while enjoying the extra space and peace of mind.
Which, of course, brings us back to the amount of stuff we own to begin with. The more things we have, the more we are going to see and think about, even if it's just for a fraction of a second. Even the knick-knacks, which generally require little to no upkeep, are attention-grabbers, which is why I like to keep them to a minimum. The fewer things you have lying around, the more you will free up your mind to focus on real living.