Check that you action is not too high (i.e. the distance from the strings to the fretboard). A lower action requires less force for pressing down the string.
Try playing for instance scales up and down by keeping the thumb free without touching the neck. Stabilize the bass with your left arm when doing this. This practice requires you to press harder with your fingers and thus will strengthen them. This technique is of course for practicing purposes only...
Play very close to the frets, avoid playing right between them. That requires less force and it gives you a better sound too.
Practice scales, arpeggios and different exercises, like the spider (see my post in the thread
hand stretching/strengthening)
...and the exercise I mention in the thread "how to get even plucks?"
How to get even plucks?.
Good luck with practicing!
