To do a preorder, just put "preorder" in the title and then add the amount of inventory you will have when the item comes in. Or if you're doing a preorder to find out how many pieces to order, just uncheck the inventory box. This will make the inventory Unlimited. Then when you're ready to decide on inventory, check the box and put how many you have left. For example, if you sell 100 pieces, then decide you're going to order 500 pieces, when you go back into the product to take it off unlimited inventory, you would put an inventory in of 400.
Something else that helps. In your sales area you can search "Negative Keywords". Basically if you do a preorder, your sales area will be bombarded with orders you can't ship. But if "preorder" is in the title, you can type "-preorder" into the search box and it will return all orders without preorder in the title. Here's a blog post about this feature.
And lastly, if your product has digital attached to it, or will only be digital, you'll want to use our Digital Street Date module to delay the download of the digital file till the release date. Please check out this help doc on how to do that.