I've been using ephemeral_response[1] in Ruby, there may be a similar library in whatever language you're working with. The idea is that it makes one real call to the service and caches it for a specified amount of time as a fixture. That way you don't constantly make calls against e.g. the Twitter search api, but when they change how it works in 6 months, your test will fail.
[1]: https://github.com/sandro/ephemeral_response