As soon as you realize your growth rates are higher than expected, you need to plan to buy or allocate more disk space. There is no way around this—more data means more disk. You can delay the inevitable for a little bit with better compression, but this is not a long-term fix, and it can impact query performance in different ways (trading CPU for I/O).
Once more disk is in place, you can plan your growth better.
Click through for some guidance on how to plan that growth.