I will try to keep this short; Look at a lot of really good design and art, a lot of it, and have the concept of a "good" piece of design in the back of your head. Limiting chaos, or at least, control of it, and heirarchy of forms, is the biggest thing, and you will see that quality in a lot of the good designs.
It's a frame of mind that you need to adapt more than anything. There aren't any actual rules (other than
basic design principles, which you should take a look at), besides "making things work". Once you develop an OCD for the things which add unneccessary chaos to works, and can recognize when something is not right in a design piece, you are ready to start trying things yourself. You should try picking apart design peices in terms of their design elements; look at the textural quality of a grungey piece of artwork, the heirarchy in an eccelent typographic poster (or one that LOOKS good at first but really has some issues that you can see when inspecting closer), etc.
Never remind yourself that this is your work. Always look at your work as if it was someone else's which you've been asked to critique.
Get really anal about alignment, proportion, and hierarchy.
Fantasize about good design.
"Less is more" is a good phrase to hold in the back of your mind.
Never accept that you have made a "perfect" design, always look for a different formulation of the elements on a page, and think of the "bigger" concepts that could be represented with them.
Start off by making something you think is good, and let people critique it, if you really want to start off strong. But you must accept that people who want to help you are actually going to pick apart things beyond your comfort level. You have to accept, and embrace this. And you have to do this to your own work too. Looking at photoshop files of Mollos and whatnot is excellent for technical skill training and finding out ways to utilize your tools. But unless you are actually dissecting the decision making processes made in the first place, you aren't learning all that you could be from picking apart a photoshop doc.