Here's something I've been doing at the beginning of lessons lately. I play a little 5-note pattern and have the student buzz it back to me.
Then, after 5-10 of these patterns, I do the same thing but have the student play them on their instrument (I have them look away so they can't watch my slide).
I've noticed that, when buzzing, a lot of students leap too far and then have to "snap back" to the correct note. I sometimes see this when they are playing lip slurs, too.
Here's a link to some of the patterns I've been using (pdf file)
Try doing these with a buddy. If you're practicing alone, see if you can sing or buzz them accurately.
LISTEN TO THE DETAILS!