When customer's release a draft RFP, it makes it considerably easier to prepare ahead of the final RFP release. When they don't release a draft RFP, it is still just as important to prepare ahead of RFP release, but much harder to know exactly what information you need and what format to present it in. This alone is a big reason why many companies make the mistake of waiting until RFP release before putting pen to paper. If the customer doesn't release a draft RFP, you should consider making one yourself.
The exercise of creating your own draft RFP will force you to seek answers to key questions. How extensive will the scope of work be? Will there be a past performance review? Will it be a "best value" procurement or a price shoot-out? What will be the most important evaluation considerations?
You'll need to ask the customer these questions. In fact, you'll need to ask more than one person at the customer, since one person won't know all the answers and some may have conflicting opinions. Don't worry so much whether their answers match the final RFP exactly --- the differences can often tell you as much about the customers actual preferences and level of internal consensus as would similarities. The key is that it gets you talking to the customer and helps you develop an understanding about what it is they want, separate from what makes it into writing. You should specifically target developing an understanding of what problems the customer would like to see this procurement solve.
Going through the effort of creating your own draft RFP also gives you a rallying point for collecting information. For example, you can begin to identify relevant projects and collect contact data based on a high level understanding of the scope of work. You can begin determining your win themes as soon as you develop an understanding of the customer's evaluation considerations and preferences.
You can also begin doing competitive assessments based on your strengths and weaknesses while you can still ask the customer about what they consider important and their familiarity with your competition. And don't forget to do a critical self-examination. Does the client know you? What is your past performance record? Get this directly from the client and not from your project staff. Don't drink your own bath water --- no one wants to admit when they've had problems with their customer. Identifying problems early give you a chance to correct them, enables you to prepare your win themes and written material as a defense, and in the worst case might lead you to walk away from a bid that you cannot win.
Finally, make sure that your internally developed draft RFP is a written document, and not just someone's "understanding" of the customer's requirement. A written document can be distributed and the level of customer understanding assessed. It can be verified with others who know the customer. And when the final RFP is released, it can be compared, telling you what to keep, what to change, and providing additional insight into the customer.