I'm going to assume you are talking about the post game. It is extremely difficult to run out of money in the main game, but here are a few neutral tips.
Get the Amulet Coin. This can be found in the building on the left on the road furthest to the north of Castelia City. Attach this to one of your Pokémon.
Then go and battle trainers. This gets you EXP and cold hard cash. With the levels you have gained, you can rematch the Elite Four. This gets you more money. And with more money you can....well buy more things.
In Castelia City you can buy a Casteliacone for ¥200. In the trailer on Route 5, you can sell the maid Casteliacones for ¥2000. A profit is made of around ¥1800.
Hope I helped!