I think if you're scoring b/w 45-49 on quant, you're probably not ready to move up to 51. 51 is really, really difficult to achieve. A lot of the quant gurus on here aren't even getting 51 on the real thing. I would focus on consistently scoring 49 first. Then try to bump yourself up to consistently 50. Once you're there, then you can try for 51, though I'm not even sure it's worth it...
To do this --
MGMAT and focus on minimizing mistakes. if your scores have that much deviation, it probably means you're making a lot of careless mistakes and you need to concentrate on getting all the basics down.
MGMAT books will help.
For verbal, again focus on consistency. If your scores are fluctuating that much, you're nowhere near ready to improve your score. Just focus on scoring consistently on the high end of your range first. Then try to improve your score.
SC -
MGMAT should give you all you need.
RC - Read proper English and understand (WSJ, Economist, NYTimes, etc.)
CR - PowerScore GMAT CR Bible
Great advice zoinnk. What do you suggest for people scoring on the lower 40-levels with test date approaching soon