One of the most valuable abilities you can use in Pokémon Brilliant Diamond and Brilliant Pearl is the TM 94, Fly. With it, like HM, you will be able to instantly travel to the cities you have visited earlier in the game, greatly reducing your travel time. But first you will need to find this article. Here's where you need to go to find TM 94 Fly in Pokémon Brilliant Diamond and Brilliant Pearl.
You will be able to access TM 94 when you defeat the third Gym Leader, Maylene. Immediately after leaving the gym after this encounter, you will encounter Dawn, who informs you that Team Galaxy has stolen her PokéDex and that she needs your help. You can help her after retrieving your Pokémon at the Pokémon Center in town. You can find it on the north side of town, just outside the Team Galactic warehouse.
After defeating the two members of Team Galaxy, you'll return Dawn's PokéDex to her. She will leave the area, thanking you for your help. Next, you will need to enter the warehouse that the two members of Team Galaxy were protecting. Inside you will find a Poké Ball on the ground and it will contain the TM 94 Fly. You can now use it while outdoors to travel around the city you've already visited, and since you've defeated Mayleen, you can use it anytime you want to use it.