One way to enforce it is to get your cheeks puffed out and then exhale completely through your nose. Then you can try doing it with a small constant flow of air by placing two fingers against your lips to restrict the flow so it's easy to puff your cheeks out. Let the air flow through your lips with your cheeks puffed out until you are almost out of air, and then extend the flow of air by thinking of squirting the rest of the air out of your mouth by bringing your cheeks in. Make sure that you are really closing off the back of your mouth when you do the squeezing part. Then the next step is to do the same, but as you run out of air, close off your mouth, squeeze your cheeks in and breathe in quickly through your nose.
Some people recommend practicing by blowing bubbles through a straw in a glass of water.
With a small diameter straw there may be enough resistance to make it easier to master. The main point of this technique is that the bubbles will confirm whether you are keeping the flow going. In general it is easier to circular breathe on the didgeridoo than on most other western instruments. This is because you are not too concerned about whether your embouchure changes as you do it, causing fluctuations in timbre and pitch. Everytime you circular breathe, the sound coming out of the didgeridoo wobbles.