Make sure your saddle height is correct, there are a number of different methods and there is some leeway, but if it's too low, you may end up pushing bigger gears more, and if it's too high you may end up bouncing in the saddle often.

single legged pedaling helps. Just unclip one leg and pedal with the other on a fast straight smooth section of road. Try to keep your cadence the same (70 or higher) for about a mile, then switch legs. It's even easier if your bikes in a trainer.

Another drill is to do rides only in the small (or middle if you run a triple) chainring.

Finally, riding a fixedgear bike with a fairly low (42x17) gear will do wonders for developing a good spin.

Most important thing is to just keep at it. The more time you spend riding, the better you spin will become. Just make sure you are mindful of it, and it will develop over time.