As I wrote in one of the posts about auditioning, it was during an audition that I realized my tonguing was indistinct because my tongue had somehow moved too far back in my mouth. I immediately started working on correcting this. One of the most useful things I found was Jeff Nelsen's Morning Routine because it includes exercises that just focus on tonguing. Attacking notes repeatedly, I was able to concentrate on what my tongue was doing and move it to a better place. I still make a point of checking where my tongue is, but that issue has improved a lot.
Another problem I had since returning to the horn was intonation. This had always been an issue for me. My horn has a few notes that are almost impossible to get in tune with the slides or by lipping and using the hand. But I've also always felt that I just didn't have a very good ear. This was a problem, especially when I started playing more chamber music. So, I asked for a tuner for my birthday this past August and my husband gave me a wonderful tuner-metronome combination. I started using the tuner when I warmed up, just turning it on and looking at it without attempting to change the pitch. I also used it when practicing specific music to check pitches and adjust by fingering or lipping the note. To my surprise, after using the metronome this way, I started playing more in tune. What's more, I started hearing intonation better. I now know where the pitch should be and I know what notes are likely to be out of tune on my horn. This was amazing to me because I had had this problem since I was a kid and I thought it was an intractable problem.
Making progress on the intonation front gave me the impetus to try to fix other problems. I have never been able to double or triple tongue very well. I single tongue extremely fast, but it would be useful to be able to double and triple tongue. I had never worked on it much as a student, but now I have begun practicing double/triple tonguing exercises on a regular basis. I still don't feel confident about using either in performance, but maybe I'll get there yet.