Here's an aphorism that I had to learn the hard way...
Your employees are already motivated - just not in the same ways that you are As an entrepreneur, I have boundless energy to invest in my business because it's my life's dream. That motivation makes it possible for me to invest all my time and money into turning that dream into reality, without ever feeling burned-out. However, I've learned that it's unrealistic for me to expect the same from my employees. For them, this is a job. A good job, to be sure, where they're hopefully challenged to grow as developers, but still a job.
You see, they have their own dreams, and my dream is just a means towards that end for them. The trick to motivating your employees is to find out what their ambitions are, then discover how their plans can be melded into your vision.
I think this is why you've perceived that people "feel I am doing it for my benefit" when you try to motivate them. Maybe they feel that way because you're trying to get them to adopt your motivations. Find out what already motivates them and how that can be used to further your goals, for everybody's mutual benefit.
The book How to Win Friends and Influence People by Dale Carnegie deals with this topic quite a bit. I recommend buying a copy of this book and reading it over and over. You'll benefit in other ways as well.
In your new startup is there a hierarchy? - like project managers/ team leaders/ engineers etc etc? In that case motivation could come from a level above them. For example from team leaders to project managers, project managers to team leaders and so on. You may also have occassional meetings with all of them and get the team leader talk about interesting stuffs that other team members did, but this should not discourage others. Instead it should encourage others to do stuff.
Talk to the team leaders and find out who is less motivated in their team and find out ways to motivate them. Some things that motivate a person are saying things like "nice job on that X module", "do you think a training on this new technology could be helpful?". Appreciating a person for their work always helps the person psychologically. It need not necessarily be monetary alone.
And motivation need not be just based on an individual. It could be collective - find out ways to motivate the entire team. Organize get togethers at least once in a month and let that not just be about work. Initially it could be about work, but later it could be steered on to some other interesting stuff.