I think that the most what students remember about their teachers and classes are things they have learnt and how they have been taught (which might include teacher personality as well as different learning methodology).
Being a teacher means being unending resource of all types of ideas, applications and knowledge. Thus it is so helpful to learn all kinds of resources online, which not only give various exercises, but much more - even lesson plans.
Lesson plans online help you to innovate your mode of teaching, have a new and fresh perspective. It is almost as watching your colleague teach and learning from him/her the best they have to give. I liked this page with IT lesson plans.
I have noticed myself that a careful planning is worth while. It does take time, but it is so rewarding. No plan, means little success and a chaos in one's teaching environment.
Thus planning enables you to achieve the goals you want. If you know where you want to go, if you know how you will go there -- there is a great possibility you will get to your destination point.
Then it is easier to improvise, change, adapt, as you already know how and what results to achieve.