Here are just a few tips I've learned throghout the years:
1. Use Reference
-Don't rotoscope. It's good to look at a piece of footage of people and try to animate it. You can either use the same style of characters or change it and simply reference the movement. In animating with reference you'll build a broad animation vocabulary that will allow you to progress and make better animations.
2. Enjoy the Z axis
-Try not to limit yourself to motion that is simply left to right. Remember the Z axis. Even if you're not good with perspective just make things turn in space and move forward and backward. Don't simply scale it. Scaling looks like scaling and anybody can spot it.
3. Ones and Twos: 1 Frame can make all the difference
- Be wise with your timing. I know you said you are familiar with the principles of timing but be very aware of it. Know when a 1 is appropriate or when a 2 is appropriate. It will make the impact of your animation that much better.
4. When you're doing dialogue:
-Use the x sheet. It's a pain but it's a pain that's worth it. It helps you visualize what you're doing before you do it. Also, jot down the gestures on the x-sheet as well. It will save you a lot of time in the end. (if you hate x-sheets, I understand. I used to refuse to use them as well and sometimes still do. If you aren't working with that much dialogue don't worry about it. But if you're working with an excessive amount, I highly recommend it).
5. A Personal trick
-I like to basically do all my keyframes on a single sheet of paper as though it were a storyboard. I like to see all the arcs of an animated sequence on one sheet to see whether it flows or not. Whenever I do this my motion looks incredibly smooth. I only draw the slug/seal/stick figure this way so I don't waste too much time focusing on that but it certainly makes the final product that much better.
That's it for now. I know there are tons more but unfortunately/fortunately I must get back to work.
Hope that helps!