Another way, the less boring part imho, is simply learning the chords. I found that when I first started, chords were very difficult on my fingers - so by playing chords I was not only learning the chord positions, but also strengthening my fingers. You should focus on accuracy, making sure each string rings out clearly.
Of course, you shouldn't just pick one way - you should use all the methods in combination, and try and do 5-10minute sessions a few times a day until your fingers get stronger.