How Do You Keep Your Staff Motivated?

One of my staff handed in her resignation today. It's a sad feeling deep down inside. I have no control over salary schemes and bonuses. I only have control of job responsibilities and try to make them as interesting as possible. But when someone else offers a better package, how can I stop my people from pursuing it.

I would really like to hear from other managers on how to keep people motivated? I work in a engineering software firm - lots of pre-sales, post-sales and consulting work.

Thanks.