In general, the place in a song where I am the most likely to make mistakes is when something is changing. Some examples of important changes in a song include changing meter, feel, or dynamics. Working on these types of transitions is one of the most important and also most overlooked elements of drumming. Transitioning succefully is a skill just like anything else, and needs to be practiced and refined as such. Some examples of transitions that almost everyone could be better at include switching between brushes and sticks, going to double time or half time, switching between latin and swing, changing meter, and changing dynamics suddenly.
How to practice: Isolate the transition
There are two important ways of practicing transitions. The first is to isolate a particular transition that is giving you difficulty, put the metronome on and record yourself attempting to make the switch as seamlessly as possible. You will hopefully notice what your exact problem is and will be better able to address it once you study your recording.
Once you start to feel stronger about a particular transition, I would als recommend trying this in the context of a song. Take a song with a straightforward form like "Take the A Train" and sing the melody will doing this transition. Try doing something like playing in one way during the A sections and switching to another way on the bridge.
How to practice: Playing with a recording with transitions
The second way to practice transitions is to find a recording that you like that features some transitions. In the video at the top I am playing along with a great recording of "Star Eyes" from Pparker's Mood" that has no drums. In this version of the song the arrangement moves between latin and swing as well as between 3/4 and 4/4. Finding a song that features transitions like these and then recording yourself playing along is a great way to test how fluid your transitions really are.
To emphasize or deemphasize?
Playing along with a recording like this is also a great way to dissect how you should approach a transition musically. Do you want to emphasize the transition with a dramatic fill, or do you want to deemphasize it by moving rapidly from one style to the next so that it almost catches the listener of gaurd? As always, the answer to this question is to LISTEN. If the music seems to be going one way, then move with the music. Don't try to plan your strategy ahead of time and then force it on the music, respond to the music in an organic way. In the video I tried to illustrate both approaches based on what I was hearing. For an example of a dramatic transition go to 1:04 when I go to swing on the bridge of the melody. For an example of a smoother transition go to 2:56 where I go to 3/4 on the bridge.
Bossa Nova groove
Incidentally for those of you who are interested in the groove I am playing on the A sections of the head, I got it from "Groove Essentials" (variation A of groove 33).