Its not impossible to score above 300 marks in GRE!
But i also say its not so easy!!
To score above 300.follow as below:
1.Make sure you complete atleast 1500 gre words including high frquency words.
2.for verbal practise,allot atleast 2-3hrs daily!
3.Quant score has to be consistently 30+,whatever the practise test you take!
4.concentrate more on text completion and sentence equivalence rather than reading comprehension.
5.Pay more attention for data intrepretation problems!
6.target for verbal 140-145 and quant 155-160 for achieving 300+ scores!