Imagine you're sitting and staring at your creative medium of choice. It's totally blank, and you just can't seem to decide what to put on it. You start to feel like your mind is in a fog. Maybe you know what you want to accomplish, but you can't figure out those first steps. You get up to grab a cup of your favorite caffeinated beverage in the hope that maybe that will get those creative juices flowing. Still nothing.
Creatives the world over can relate to that scene. If you can't, then your brain is amazing. It's probably one of the most frustrating situations, and it can get pretty discouraging. It has a detrimental effect on productivity, which can turn into an extremely stressful situation.
Options have a negative effect on creativity. That sounds counter-intuitive, but it makes sense when you consider how our brains work. With the advent and evolution of computers, we now have instant access to all the tools we need. It should feel empowering, but often times it can be overwhelming to be bombarded with options. You literally have the power to create whatever your mind can think up, so you can't think of anything.
Necessity births creative thinking, so your best option is basically to force yourself to be creative. You need to box yourself in as much as practically possible. That sounds extremely limiting, but that's pretty much the point. When faced with obstacles that need to be overcome your creativity will kick into gear.
So for example, when I created the Kinston Icon Set I gave myself a limited canvas of 90x90 pixels, a small color palette, and a deadline of 2 weeks. This forced me to be creative about how I approached each design. Depending on what you're doing though, that level of restriction may not be appropriate.