You see what sells, then incorporate the features others appear to want. I assume you design in features which have proven popular in the past right?
If you ask around about what people want, you'll get a response - what that person wants, which may, or may not reflect what others want, and will typically be specific and unique to that person.
If you could get a large enough response rate - 10's or 100's of thousands, you could get a broad enough picture (customer response cards). Short of that you'll be designing a camel.