Once and again is pretty old-fashioned to me.  I would use "now and then", "from time to time", and "time and time again" for various shades of meaning.  "Now and then" means occasionally; there's a sense of irregularity about it.  "From time to time" is still not often, but there's a sense of more regular occurrence than now and then has.  "Time and time again" means quite often, sometimes too often, as in being told time and time again not to slam the door.  For the OP's sentence, I would most likely use from time to time.

What's the meaning of this sentence?
I am starting out as a freelance website developer so I need to improve my graphics drawing skill. Up till now I'm using an open source tool GIMP to create the concepts and mock ups of the websites I design.I know that Photoshop is the Industry standard(at least as far as I know).

So,what books can help me in learning some advanced Photoshop techniques used frequently in web designing.Also I want to learn how to draw cartoon characters like this.I know it is a matter of creativity but still there must be some techniques which I can learn.Please express your views.

Just for the record, I'm basically a web developer so my graphics drawing capabilities are very limited(but I want to improve that). You can have a look at this site I designed using GIMP. 
