So after my last lesson with my instructor, I've learned I'm tending to let go of the pinkie on fast singles/doubles.
I thought I was past that problem already but he pointed it out to me again. I look in the mirror and the problem is really evident on my right hand (for some reason my left hand can keep the fingers on the stick better)

I've been consciously thinking about keeping my pinkie on the stick but it still refuses to go. I thought about another method: take a string or rubber band or something, and wrap it around the pinkie and ring finger, just tight enough to hold them together but not enough to cut off circulation.

Feedback?

Theres a few things you can try that might help.

Try slapping the palm of your hand with your back 3 fingers. Do this without a stick in your hand. Mark Wessels talks about this here (about 3/4 into the video). This will help develop those small muscles in your fingers and the coordination to get those 3 fingers to work as a single unit. I like this exercise because you can do it anywhere, anytime.

You can also practice singles and doubles with your sticks backwards, holding them at the butt end and playing on your forearms. For some reason, many beginners greatly improve their technique when playing like this (not saying that you're a beginner) than lose it when they hold the stick normally.

One time I actually taped a students three fingers together to get him to feel how they should be working as a single unit. I only did it the one time and only for a few minutes. To be honest, I was frustrated with him that day. I think that method is a little extreme and may not really be effective. The pinky may just be "going along for the ride" and not really using the muscles needed. When you remove the tape (or string or rubber band) you may be right back where you were when you started.

As always, take it slow and practice it a lot so you can develop the muscle memory and coordination in those fingers. Don't forget to be patient with yourself, these things take time to develop and even more time when your trying to break bad habits you may have developed over the years (I know that part first hand, trust me).

Hope this helps.