Bobby Chiu is another artist who is a saviour for lesser artists such as myself :P

One of his latest tutorial videos featured a technique that is so useful you would not believe it. It basically involves opening up another widow of the image you're working on, by going to Window->Arrange->New Window For "image name here".



As you can see here with my painting of Sphinx, it opens up another separate window of the same piece of work, which you can then shrink down. Most artists usually zoom in and out as they paint, to try and get a feel if the painting works on a whole. Bobby Chiu's advice is that if the painting doesn't work on a smaller scale, but does on a larger scale, or vice versa, then your work is out of balance, and there for wont work as well in the end.
The other fantastic thing about this technique is that you can paint in either windows, and the result will show up in the other window.

I highly recommend this technique to any digital painter- it is incredibly useful, and can really help you sort out any issues you have with your work.