1) Read up stories about innovation and how companies innovate ideas. One needs examples to replicate. Preferably successful stories sharing how they've done it.
2) Depending on your field of business, get magazines related to it so your team can get latest ideas. (i.g: The Economist, Harvard Business Review). Even surfing the latest columns in websites sharing the latest updates in that field.
3) Network. Get them to network with other companies. Take them to those business conferences and networking sessions to grasp more ideas.
4) Have a fault tolerance environment. The reason why Google is so successful is because they allow their teams to come up with ideas and let em' try it out. Even if they fail they won't be penalised for it.
Zero-fault tolerance environment is toxic to relations among your teammates and even to the business as a whole as everyone will be in their comfort zone fearing of making mistakes.
When one or more of these are achieved, have a brainstorming session. Avoid brainstorming in the early morning or late afternoons as that's where most employees (and you) would experience energy slump. What I love doing most is downing several cups of coffee or espresso shots within one quick go, which will help you enter 'hyperactivity mode'. That's when you get excited and energised, and ideas will just keep popping up. Alternatively, sedative states work too - 2 glasses of wine, or taking drugs like marijuana (won't recommend it though).
But like other said, they need a 'reason' to get motivated to innovate too. Say, a reward system. It can be monetary rewards or non-monetary rewards (vacation days, a week off if it goes well, etc)