I am at your Mercy

You are not asking the investor for a personal favor, you are putting forth a business proposition. You need the money to go ahead, but that should not lead to desperation. This has to be kept in mind when you approach the investor. There is no need to be afraid of saying no when it has to be said. In fact, not saying it when needed can be a big mistake. The deal has to be mutually beneficial and in order to make it so both parties have to make compromises. What you need is a fruitful partnership where the power to take decisive decision still rest in you. If VCs smell desperation, they will push you to the limits and will end up getting much more stake in your company that they should normally get, and you will end up having much less control in your company. Be confident and be reasonable in your negotiation. Approach them as if they are an option but not the only option for you. Having some training in negotiation will be of huge advantage for an entrepreneur.