Toss it, if
You have started feeling uncomfortable in it, no matter if it's the size or the style you are growing out of. For me, for example, that's anything that's sleeveless. No more bearing arms!
You cannot remember one occasion on which you have worn it in 2011. It might come back in style, alright, but by then, you will probably also want to buy it new.
Mix and matching it with the rest of your clothes and bringing it into accordance with your own style is giving you a headache. Such items get in the way too much, especially in the mornings when dressing is also about efficiency.
It's not ideal, maybe because the fit is a little off or because of some detail you don't like. Of course, that's not yet reason enough to toss it, but if it's something you can easily replace by something better out there, go ahead and do it.
For make up, please be aware that cosmetic articles have only limited shelf-life. That is especially important for mascara and lipstick/lipgloss, but also foundations, etc. General rule is, if color, smell or texture have changed, it's time to get rid of it. Also, if you can't remember any more when you bought it, it's probably old enough to be tossed, too.
OK. You don't wear it often, and it doesn't go with most of what's in your wardrobe. But if you really, really love it, why not hold onto it for special occasions. Be perfectly realistic and honest about it, tho, and remember that you are trying to de-clutter your closet here, don't leave the job half undone! Set your mind to "de-clutter mode" with the help of this post.