For example you have two folders folder A and folder B to backup. And you want B to be backed up only when A is backed up every time because A contains the most important documents and needs to be backed up always. So you can just create a separate backup set for folder A and assign it a higher priority than B. That's it. As you can see below, I have scheduled the desktops of my PC to be backed up always. Then files from my Dropbox will be backed up to provide an additional security as all my vital docs are in Dropbox and Skydrive. Then my photos and videos from my external hard disk will be backed up.
If that is not enough, you have the additional flexibility to set the configuration for each of your backup sets. For example, you want your desktops to be backed up only during the day time and the photos and videos during night time, you can easily set the settings to do just that. For that particular set, you can also set how many number of versions of your files you want to have. Of course, for your important files, you will always want to have more number of versions, than, say, your photos and videos which might change seldom. You can also set the number of places where you want to backup. For example below you can see I backup to CrashPlan’s cloud and another laptop, for the others you can also add just to another PC or laptop. As I have already mentioned in my previous posts, with CrashPlan, you can continue to use it even without using CrashPlan’s cloud, you can backup to your friend’s using it.