Here is a simple exercise to get your diddles smooth and even*. In this case we're striving for very open, clear sounding strokes. Each note should sound the same.
Then, switch hands.
When you're feeling pretty comfortable playing this with a click or a track, add in a foot pattern of your choice, and play the exercise over it. Do it a lot. I find that if you can do the exercise comfortably and with no mistakes for 40 seconds or more, you probably are ready to move a little faster.
The next step is to work this thing up to lightning speed, staying relaxed and using good technique. You'll find that there will be a certain tempo plateau that will force you to use a bounce stroke for the second note. Once that stroke feels comfortable, you'll be able to get fast very quickly.
This exercise can also apply to your feet, whether on double kick or using a single kick and hat. A double stroke in your feet is also a fantastic tool to have at your, uh, toe-tips.
*One exercise alone will not produce good diddles. Continue to practice exercises to develop your basic stroke with proper technique, and find ways to apply diddles in your playing. If you do it consistently and over time, you'll find diddles popping up in your playing quite naturally.