Okay, so you've committed yourself to the discipline of practicing every day for at least 30 minutes. But what do you practice? And how do you do it? (I'll probably spread this over a few days.)
Actually, if you just played anything you wanted for at least 30 minutes a day, you'd make good progress. But it's better to have a clear plan, or even a regimen, if you really want to make the most progress. I remember reading something from Dan Crary in _Frets_ back in the 80s about this topic, and I've talked to classical musicians about their regimens. (And what they do is incredible!) The key, I think, is to focus your attention on just a few things at a time, concentrate, and use the time wisely.
I like to warm up for five to ten minutes, with scales, patterns, all downstrokes, tremolo, or whatever I feel will warm up my muscles and get me going. Doing this with a metronome is best (more on metronomes later). Then ten minutes on specific skills: right hand exercises, triplets, tremolo, left hand workouts, etc. Then the rest of the time on a specific tune (right now I'm working on the Monroe break to "Little Cabin Home on the Hill"). If there's any time left, just play a few tunes for fun that I like, or run through a few original tunes (doesn't everybody write tunes on the mando?) It's best to do this with no distractions--no TV in the background, in a room by yourself so you don't drive the significant other crazy.
That's thirty minutes well-spent, focusing on particular skills and tunes. If you do it every day, it really will make a difference.