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It's hard, and will take decades. That said, there are a few things you can do relatively easily to make a start.

1. get rid of zoning laws that prevent building multi-family units.

2. invest heavily in public transit. Buses are great if they are widespread and arrive frequently.

3. in areas that have significant traffic, use trams/subways instead of buses. They carry way more people and don't sit in traffic.

4. make city centers dramatically more walkable. A 4 lane street is much harder to cross than a 2 lane street (or 2 one way 2 lane streets).

5. don't build new roads/highway exits that will encourage further sprawl.