HELP! going from 48 to 50+?

A couple of things:
1. At this level, exposure becomes most important. Hard questions are hard because they are presented in a way that it is more difficult to figure out what they are trying to test you on ...(since the topics tested are the same at all levels, they try to veil the concept in more layers as they go higher up the difficulty level). If you work on many hard questions, questions will start clicking quickly. Those questions are not more time consuming; it just takes time to form the link. Also, try and find the most efficient way of dealing with every question to have plenty of extra time for the more 'innovative' ones.
2. Trying to go from 48 to 50/51 may not actually be worth your time if you are not at the same level in Verbal. If you are below 40 there, try and put much more effort there since the scope of improvement is substantial in that section. If you are around 44 in Verbal, then just practice for as long as you have. You should get a great score!