This is a good question Jef and i would like to answer you accurately. First of all you have to understand that employees are diverse in terms of their interests, needs, values, cultures, psyche and experiences. This means that you cannot apply a single motivation formula across the board.
To know what can motivate a particular employee you first need to understand that particular person. understanding his or her past and present situation is of critical importance. Ambitions, preferences and values should not be overlooked. It is from here that you can be able to gauge an appropriate way for motivating.
Money can motivate if the person is money hungry. However empirical evidence cautions that this is for a limited period of time because an optimal point is reached when money can cease to be a motivational force (psychologists call it satiation). This shall require you to device another alternative source of motivation. Thus motivation should be best be contemplated as a process and not an event.
Challenge is the second source of motivation that i would like to talk about. Human beings like achievements because it earns them recognition among colleagues, seniors, professionals or stakeholders in the industry. Example, an Executive who steers his firm to exceptionally higher levels can be recognized by other management professionals as well as investors in the sector. When we are recognized we feel happy and more important about ourselves. So if an employee is of this type yearning for recognition then you need to give challenging (but not impossible) tasks upon whose successful completion a form of recognition is accorded. Recognition can be as simple as a public vote of congratulations up to promotions. Challenges can be set in form of performance targets, promotions to bestow increased responsibility and so on. Challenges makes the job exciting. When one overcomes one challenge he becomes eager for another one. Failure to recognize employee achievements is highly demotivating.
Thirdly, make your job exciting. There are several ways in which you can make employees' job exciting like providing new methods (example computers) of work, transfers and job rotations, organizing surprise parties for employees, delegating responsibilities and such.
Thanks Jef i hope you are answered. Be free to ask for more or clarifications.