At-will termination. Finally, some terms of service have a clause that states they can pull the plug on your account, just because. Don't be surprised if you see something like this -- it's usually in there as a catchall way to kick people off if they flaunt the rules or consume a disproportionate amount of the service's resources. You may not need to worry about this most of the time, but it may be used to justify booting you off if, for instance, you use an unorthodox or unapproved method to retrieve or mirror your data. Google has this clause in paragraph 4.3 of its ToS; Yahoo's ToS has it in section 15. In both cases, it's worded in an open-ended enough fashion to make it possible for an account with either service to be closed for no apparent reason at all.
Create an exit strategy
If you don't have major qualms about a service you're with but you still want to create an exit strategy, a few basic points are worth keeping in mind.
Keep local copies of everything that's crucial. The only storage you can completely trust is the storage you physically own, so always make sure there's a local copy of everything important. If you've already been trusting your only copies to a site, break the habit now. Any Web service should be thought of as a replicator, not a repository.
For instance, don't ever trust a remote service to your only copy of a given photo, since the service's rule about data preservation might not be in your best interest. Flickr, one of the most popular photo hosting services, doesn't allow you access to the original copy of an uploaded photo unless you have a paid account. A utility like Flump or FlickrEdit can help you extract pictures from your stream, although they will probably not be able to rescue images that aren't publicly accessible. (Flump, in particular, requires a Pro-level Flickr account to be useful.)
On the other hand, many Gmail users have no qualms about leaving their entire trove of mail on Google's servers -- even though both POP3 and IMAP connectivity exist for Gmail, making it not only possible but easy to keep mail local. It's easy to get into the habit of unthinkingly trusting Gmail to always be there -- at least until the next network outage or Google cloud failure.
Practice making local copies of the service's data. If a site has a way to allow you to make a local copy of your data, make a practice run. Step through the process of creating a local copy of the data and see how difficult it is -- how many steps are involved, are third-party tools required and so on.
Also be warned that the process could change on you without warning, so you should take a full review of the process every so often or whenever you get word about major changes to the service.