To start with, it's always advisable to collect as many previous question papers as possible, solved or unsolved. Having gathered the knack from the solved ones, try solving the unsolved ones, making a pragmatic assessment of the number of questions you can solve in the given time frame. Always start with the easier portions followed by the not so easy ones. If you get struck with a question, it makes better sense to move on to the next one.
Remember, your timing is very important, since you are required to solve the maximum number of questions in a limited time. Once you are well aware of your strengths versus your weaknesses, get as many mock tests as possible, improvising your score with every consecutive drill.
Invariably, aptitude is all about solving real life problems. Therefore, try your hand at solving crosswords and number puzzles whilst considering different possible angles to a given problem. Give it a shot with "The great book of puzzles and Teasers" by George Summers, an essential tool for trying your numeric and verbal skills.
Writing and conversation skills are no less important. Though you needn't be a Pulitzer essayist, you must be able to express yourself in lucid English. Build your vocabulary by reading a lot and observing how the same word may be used in different contexts. Print and electronic media go hand in hand toward accomplishing such skills.