Your greatest responsibility is to your team. You should know each of them well. You should stretch your team, but not overburden them. You should usually talk to them about the way they are being stretched. If they buy in to it, they will be well motivated. On each project, or every other project, try to stretch them in both a way that they suggest and a way that you think will be good for them. Stretch them not by giving them more work, but by giving them a new skill or better yet a new role to play on the team.