Providing checklists or buying-decision matrices that help business managers look at tech companies and figure which are safe to hire and which require more investigation is also a winning strategy, Cramm says.

So is letting some groups rent what they need with minimal interference from IT, she adds, such as developers, marketing teams and others that might have an intense but short-lived need for more capacity to meet project deadlines or test new products.

Informal buyers are taking over big parts of the IT buying process not because they want to, but because IT isn't giving them what they feel they need as quickly as they think they need it.

"Users are saying, 'If we think you don't offer good collaboration services, we can go outside for that. If you don't have easy storage options, we can go to Dropbox. If your instant messaging is crummy, we'll use Skype," Schreck says. "IT doesn't really have the ability to say 'No' to a lot of things anymore. Users can just go around them."