You may even not know that your nozzle is partially clogged. Eric William made this excellent tutorial on how to fix partially clogged nozzle and improve 3d print quality.You will need acetone and some tools. But it looks simple to do.
Thank you Eric!
Source: https://www.youtube.com/watch?v=SfACwC9diQY
Update (27.9.2014.):
Here is another video tutorial and method for unclogging the nozzle without disassembly with detailed description at:
http://bukobot.com/nozzle-cleaning