On chromatic, I focus mostly on scales, learning "Heads", and reading (music).
On diatonic, I've been focusing on certain techniques: blow bends, chugs, and whoops.
For those of you who are engaged at any level with learning a skill, I want to share with you the words of a teacher of mine:
"Practice does not make perfect; perfect practice makes perfect"What does that mean? Just this: Go as slowly as you need to, as carefully as you must, to learn the skill correctly. If you must learn a foundation skill first, do that. Even if you must go very slowly and think about each step as you take it in order to do the thing correctly, that's ok. It's not a race, it's a practice. Go easy, gradually build on what you learn, and practice the skill in a way that allows you to make fewer mistakes as you go.
Most teachers will tell you that you should only practice a particular skill or rudiment for a brief period, say 10-20 minutes at a time (I shoot for 15 minutes). Working too doggedly or obsessively on a single action can burn you out. Break up your practice. If you're working on bends, alternate that with working on chugs, ear training, and even reading. Keep your sessions fun. Oh, and it's often helpful to record your practice on rudiments, in order to hear and measure your progress in a detached way. Practicing in this way (small bites, detached review) can be a very positive experience if you remember to take your encouragement from small improvements.
Remember that your instrument is the most portable one there is, aside from the human voice. You can easily tuck a harmonica in a pocket or purse (inside it's case, though!). Take it with you, and if you have one of those inevitable little gaps in your day, have a little harmonica break - it'll help your smile-age! (But NOT while you're driving, of course!)
Finally, I'll remind you that the best players are outstanding listeners. I know from personal conversations with some of the best players in the world that they listen in a much more detailed way than novices do. Strive to listen better than you play - listen for the details, nuances, even the suggestion of a hint of a nuance...