There's those hard times where you struggle for money, well with some luck this post shall be able to enhance your pockets and options!
1. Island farming - Island farming is the most common among players who've completed the storyline.
What will you need? - You'll be needing a Vs Seeker which is obtained from Vermilion City just inside the PokéCentre, you'll also may want a Amulet Coin which is acquired from Prof.Oak's aide which is found in between Route 16 and Celadon City however you'll need to have at least 40 Pokémon caught on your PokéDex, and finally you'll need a high enough Pokémon to sweep the trainers.
How do I do it? - you'll need to have your high level Pokémon which has the amulet coin held to start with and you'll then need to search the islands 5,6 and 7 for all of the trainers, you'll need to activate the Vs Seeker to re-battle them.
The estimated amount for each run which can only be done once every 6 hours is 100k.
2. Everstone farming - for players who've not got as far as completing the league.
What will you need? You'll first need a Pokémon level 15 with thief which Wooper is a suggested Pokémon for the job, you'll also need Rock Smash, this HM can be found on Island 1 in the Ember Spa.
How do I do it? you'll need to teach a pokémon Rock smash (and surf for the travel there) and make your way to to Island 1, once reaching there you'll need to head right, surf and head up, once you keep going up you'll reach grass with a lot of rocks that can be smashed, you'll need to smash them and thief the Geodudes that appear (suggested having level 15 thief first in party), Geodudes have a 5% of holding a everstone, however they also have a 5% chance of holding Old Amber, Helix Fossil and a Dome Fossil which increases your looting rate.
What do I do once I'm farming - Once you're farming you'll notice you'll faint a fair few times but fear not Ember Spa's water acts as a healer and heals the whole team allowing you to return to your farming, when you're finished you can sell your loot on the Trade channel for roughly at the prices of 5/10k per Fossil and 15/20k per Everstone.
Warning if your Pokémon is holding a item it can not take the item the Geodude is holding.
3. Pay day - Pay day is also another common method but requires the time and patience or money to acquire a high enough Pokémon for the job.
What do I need? - You'll need a Pokémon with payday which I suggest Meowth or Persian for people newly trying this; you'll need to level the meowth/persian with pay day to a decent level (higher the better income you'll make).
What do I do? - Pay day is a move that gives 10 times the enemy Pokémon's level into cash (Example: Gloom level 32 will give $320 at the end of the battle) Which maintaining a constant farm will hook you a respectable amount.
I do apologise for only giving 3 examples and will be extended if found useful.
MyMrGoblin