baKa-neKoh - I totally agree with you on taking breaks every once in awhile! Sometimes I'll draw and draw for hours, feel all good inside, go eat dinner, then when I come back I'll wonder if I was on crack while drawing it

Like you said, somehow when you look at something for a long time you just get accustomed to it.. so you won't see what you want to fix. To prevent a lot of frustration, take little breaks after you've drawn some.
This might just be me, but I really like putting on music while I'm drawing.. it's relaxing, fun, and you can get in the mood of what you're drawing! Be careful though; once I started drawing a really happy picture while listening to sad songs, and somehow the picture turned into some kind of angst-romance thing..
One thing that can really help and kill at the same time is practicing something over and over again.. uh, this sounds weird, but when I was in school I ALWAYS doodle on handouts and whatever I have in front of me, and somehow I got used to only doodling faces on ONE side.. so, I got really good at drawing on that side, but when I tried to draw from the other side - it looked so much off..

Also, if you're running out of ideas for what to draw, don't strain yourself to come up with ideas. I get "artist's block" a lot.. sometimes I'll try to draw for months at a time and NOTHING will come out.. if this happens I just don't think about it.. I look at other artist's work as motivation, read stories, just do normal things.. and then there's always something that will spark, and I'll be motivated to draw again

One last thing, I find that drawing things that you have experience with, or things that you've experienced is like writing in a journal. I really love drawing and I usually don't like to talk or write in a journal about my feelings because it just feels awkward to me

So, I draw.. sometimes I'll feel so worn out from drawing out my feelings that I'll just go to bed and the stress will lift a little, or my feelings will have been put into a drawing and the same effect.. so if you're ever feeling real sad, or real happy, try drawing it out