It depends what game you're in. You can go underground on Diamond/Pearl/Platinum and get loads of items like stones then sell them do pokemon leuge over and over again.
HeartGold/SoulSilver, if not already, use mum's savings makes sure you don't run out. Sell items like nuggets and rare bones. Do pokemon leauge you get like £10,000 each battle.
Black/White, sell items to guy in poke center in i think its either the ice place forgot name or dragon place. Also forgot name (basically last to gyms) he buys your items for like twice the amount anyone else does. Do pokemon league. Dive down to the caves to get all of the items, sell them for around 10,000 a piece (only after pokemon league sell to the person in the mansion).