How Do I Prevent This from Happening?
Go responsive. The best solution to this is to have a mobile responsive website, Google will see that your website works nicely with a mobile device and eventually begin to rank your website higher than those that are not responsive. Make sure that all of your relevant content is visible on the mobile device too.
When Will This Happen?
As per usual, Google are cautious about release dates, however they have said that they are heavily testing in areas before they release it to a specific region. In their May hangout they have stated that there is no exact date, but due to the high number of mobile users it will definitely be coming in one form or another.