This will not help you seasoned sewers out there, but for the newbies:
To get your project looking really professional with crisp, clean lines ensure you always use your iron! Iron seams, corners, everything. By the time I'm finished a project I've ironed it countless times. I often think my iron is as important as my sewing machine, but they compliment each other very well.
Ironing can mean the difference between success or a shapeless, shaggy looking something!
Obviously, this doesn't apply to every project, it depends on the type of project and the fabric used.
Happy sewing! (and ironing!)