I find I get into slumps if I come up against a higher level. For example, after about a year of playing, I sort of lost interest, I think it was because I wasn't good enough to play solo bass stuff, but too good to play the boring crap I'd played before. I think finding new music helped me there.
Now, I think I'm sort of in one, I find making a practise routine, and sticking to it, can help. I think my challenge now is ear training, I'm just finding it so hard. To give you an idea of my current level, playing each of the modes of the major scale in C, they sound the same to me. I was very proud when I managed to tab out a Strokes song. I think if I can reach the level where I can just play along to whatever's on the radio, I'd be so much happier at bass. But I can't seem to get to that...