If you have certain template compositions that you would like to reuse, consider saving them in a project and then importing that project into each project that you're working on. Then you can have a library of resources in your Project panel for reuse. You could also look into template projects. See "Projects".
At the more fine-grained level, if you have certain sets of effects and keyframes that you use a lot, consider saving your own animation presets.
Also, in addition to Todd's suggestions, remember that you can simply duplicate layers and comps and replace footage at any time by Alt+dragging items from the project panel onto the timeline or using Ctrl+Alt+< to replace them. If you wanna go all fancy, it might even be possible to automate this to a point where it's all rigged up with expressions and you only define the critical points e.g. with a marker....
Thanks guys, I will give all your suggestions a try