The secret of the Warner Brother's DCAU (DC Animated Universe) was the consistency of the teams who developed the shows. Bruce Timm and Paul Dini were powerful and instrumental forces in the development of the themes, characterizations, character designs and overall flow of the early series. Their influence was so powerful, the series are lovingly called the Timmverse or the Diniverse.
- Batman, the Animated Series was highly acclaimed and spun off a variety of series with the same development, writing and production teams.
Batman has been consistently ranked as one of the greatest animated series ever made. It has been highly praised for its sophistication, mature writing, artistic ambition, and faithfulness to its source material. In the 1992 year end issue, Entertainment Weekly ranked the series as one of the top television series of the year. --Wikipedia > Batman the Animated Series
- Superman, the Animated Series featured a similar art style and was produced for the WB Network. Since the same writers were involved, they made sure to utilize the same timelines and events as anchor points of the stories. They also kept the villains and stories consistent between the two shows, treating them as if they were part of the same continuity.
- The New Batman Adventures were aired alongside Superman the Animated Series, again allowing tight continuity because both shows were running simultaneously.
- The two animated series eventually were bound together as part of an hour long program The new Batman/Superman Adventures.
- Within two years (1999), the release of Batman Beyond was released on the WB. Wildly popular this show kept the momentum of the DCAU and eventually led to the development of the Justice League. The Batman of the Animated Series became the same Batman of the Justice League.
- Again, its consistency with the continuities before it gave a feeling of continuing the previous works and the Justice League continued and was eventually replaced with the larger and more extensive Justice League Unlimited.
These shows were the perfect storm of animation. A wildly popular first show, Batman the Animated Series, won four Emmys and was nominated for six others. This showed there was both a hunger for the work and that this was a team worth keeping in action. Each success bred further successes and this prompted the entire team to remain aware and focused on keeping their continuity clean, functional and easy to follow.
You can find a listing of the DCAU timeline and how the shows fell on that timeline here. It is unofficial, but it shows a very firm awareness of how they wanted to portray their universe. I am certain the show producers kept a similar tool/service for their own work.