First, learn to recognize good design. Some people claim that you can't learn good taste, but I disagree. You just need to pay close attention to everything you see on a daily basis and store it in an accessible part of your brain. Most people don't store design information, they just store whatever was communicated.
Formal training can be of tremendous value here, but in this day and age it's also viable to be completely self taught, especially with some good books.
Once you have an eye for design you can apply the basic principles as described in other comments here. Without the eye then the principles are meaningless. You also won't learn much by looking at your own work. You're too close to it, so objectivity is all but impossible. You need to leave it alone for a month, absorb as much design training as you can, then come back to it with a fresh eye and quickly critique it before miring yourself in the details again.
Comments
First, learn to recognize good design. Some people claim that you can't learn good taste, but I disagree. You just need to pay close attention to everything you see on a daily basis and store it in an accessible part of your brain. Most people don't store design information, they just store whatever was communicated.
Formal training can be of tremendous value here, but in this day and age it's also viable to be completely self taught, especially with some good books.
Once you have an eye for design you can apply the basic principles as described in other comments here. Without the eye then the principles are meaningless. You also won't learn much by looking at your own work. You're too close to it, so objectivity is all but impossible. You need to leave it alone for a month, absorb as much design training as you can, then come back to it with a fresh eye and quickly critique it before miring yourself in the details again.