Hi,
The most important thing is components placement. More time you spend on component placement, the better PCB you get. The measure is, if you spend 90% of time on placement, and 10% of time on routing, then you should get a very good PCB design. But if you have to take more time on routing than this, it means that your placement is not good (from my experience). So you should go back to do placement again, and then re-route the board. When this condition meets (that means if after placing all the components, the routing is very easier), your PCB design is perfect. Other things also matters, like, you should keep the Analog and Digital grounds separate.
You should read lot of documentation on this subject, if you have decided to make career in this field. I have just put some words here from my experience. Best of Luck.