Your methodology will depend on the product or service you plan to offer. If it's software you could build an interactive mockup. If it's a physical product you could hire a 3D artist to create some photo-realistic renders based on your product specs. The rest really comes down to having a strong knowledge of your market and your offer. Like Elyrly said, you could go the crowdfunding route. You could also set up your own landing page and accept email addresses or pre-orders. The key is to make sure you have a strategy and system in place to actually fulfill your promise. That might mean lining up manufacturers or other vendors ahead of time, creating a budget, and working out a timeline for fulfillment.
I have seen products created on crowdfunding website that actually showed how-many customers have paid for the item. This in turns allow you time to manufacture the product knowing each item is not a sunk cost.