A lot of it is picking the line of least resistance/likely death. If you want to roll down on both wheels you need to make sure there's room for your front wheel to roll forward and allow the back down otherwise you're likely to go over the bars or lose the front end in a chasm of death. It's better to build up the confidence and go down controlled on the back wheel in stages, caefully placing each wheel on take off and landing. Failing that the next best option is to go down sideways either backwheel then front wheel or together in a sidehop down type thing... Final option is to go down backwards. May sound weird but on a big slab or steep slope if you have both wheels on the ground and inch down backwards you can keep your weight over the front and trust the rear tyre to do the gripping. Experiment and see how you feel.