One exercise we used to do when I was a drumline instructor was to focus in on each of the sixteenth-notes. I'll use a phonetic description (1-e-&-ah) to make it easier to understand.
If we're in 4/4, you'll have a total of 16 sixteenth-notes, right? Now I'll start by playing (with a metronome) the first beat in each grouping, ie. 1, 2, 3, 4, making sure to give the note its proper value as a 16th-note (not quarter or eighth). Then after four times you'll play the next 16th-note over which would be called the "e" of every grouping. After four times, you'll play the next one over, the "&" through four times. Then you'll play only the last note of the grouping, the "ah" four times. After that, you go back to the top so at the end of the "ah" sequence, you'll have two 16th's together, "ah 1", right?
Metronomes help immensely here. Remember, you're only playing the notes that are being pointed to. All other notes are felt internally and are silent. Each grouping (measure) should be repeated at least four times. And each note should only have 16th-note lengths, not daaa-daaa-daaa-daaa but dot-dot-dot-dot, etc.
As always, it's effective if you use a metronome, start slowly and deliberately and work your way up to faster tempos.