If you're not at the intermediate level, the first goal is to continue to practice until you get to this level. Because there are two problems standing in your way: the choice in repertoire is very limited and you spend your time practicing only one piece/song at a time. In order for this approach to work you must be able to handle several pieces at a time (hence, intermediate level).
So for the intermediate pianist, make sure you choose your pieces according to "timeframe." The traditional approach to studying several pieces at the same level doesn't work so well, because it's like trying to climb several mountains at once. What I recommend is practicing pieces that fit day-to-day, weekly, monthly, several months, and half-year to a full year. Of course you don't have to follow this program exactly and can adjust for your own specific needs.
Why this works well is: 1. You will always be making progress at one level or the other 2. You can consistently work on new material (even if it's not that difficult, that's okay) 3. It keeps it fresh, so you're actually able to practice longer 4. Although you will have much more fluctuations, over time you end up making way more progress than the traditional way.