The problem with farming is that it's different for each person's specific tastes, very much similar to crafting.
Are you the type of person that enjoys a constant yet small income of profit, similar to Alchemy and cooking with the creation of Silent oils and various food?
Or are you interested in the chance at larger profit yet possible gaps between these profits, more similar to Smithing and Bonecraft with HQ items?
These are questions that you need to ask yourself before you step outside of town because this decides your farming location. Some are geared towards one type while others are the complete opposite.
You also need to decide how you're going to be farming. Are you farming alone, two boxing, with a friend, or NM hunting? These need to be taken into account to decide if a location or item is worth enough to split amongst friends that you farm with and whether or not to change your destination.
Do you have a craft already that you're looking to use in your money-making scheme? If so, look through your recipes for profitable synths, every craft has them. These recipes you choose will affect and direct you to what farming locations you travel too. Make sure to check the prices that NPC's have though before you farm. Some NPC's sell things extremely cheap that can keep you from having to leave city walls all together to make some quick cash!
Is your AH listing all ready bogged down with items that won't sell? Or do you have 7 slots sitting there collecting dust? Are you willing to wait for profit, or do you need instant gratification? These once again depend on you and your personality.
Make sure to go through these questions, they are very important to maximize your profit. You want to go through whichever method makes you the most money in the time spent farming. Keep a track on an Excel spread sheet, a note book, a napkin, anything! It may seem OCD but if you want to maximize your profit in this game for the time you put into it you must discover what works best for you. If you skip this step, you just tippy toe through profit and will be upset at your gains.
Now, with all this in mind, I'll give some personal examples that I use.
*The Instant Gratification Farming*
-This style expects you to NPC your items for quick gil. In the long run this isn't the best choice for most people, but a temporary patch on your wallet.
Your looking for monsters that you can kill quickly by yourself, that have a quick respawn time, items with high drop rates, and sell well to npcs.
Some examples of this can be:
1) Bugard's in Miseraux Coast. Originally much better before the nerf, they very frequently drop skins and tusks that npc for 200-800G each. They have a 5min repop timer and multiple pops in the zone. They are also located right next to Tavnazia Safehold where you cna quickly recharge your HP as well as NPC your items.
2) Flytraps in Various Locaitons. You can find these monsters throughout many zones, mainly Carpenter's Landing where they have a 5min respawn and 30+ pops! Their leaves, although a lower droprate then some other possible items, NPC for 1,200-1500 G per leaf!
3) Diremites in Pso'Xja. Your looking for primarily Avatar Blood here, a common sought out item in Weskit synths. These bloods npc for 3,600-3,800G per. The diremites in Pso'Xja also drop chips that when given to Cid, provide you with a CCB Polymer which AH's for roughly 20k.
*The Group Farm*
-This style of farming is used when you want to tackle monsters of a higher level with one friend or more, thinking you'll get better drops. Use this site as well as wiki to research. HIGHER MOBS =/= BETTER DROPS. An example of this would be Crawlers inside Crawler's Nest to Crawler's outside in Rolanberry Fields. The lower Berr Grubs have a higher droprate on silk then the Worker/Soldier/Rumbler Crawlers.
Your looking for monsters that give you drops of decent value that can still generate enough profit to satisfy two or more people. If you're able to two box, or are the sole person receiving the items from a nice friend, this MAY be the better route for your farming.
Some examples of this can be:
1) Elder Goobue's in The Boyadah Tree. These monsters are much stronger then their coutnerparts in Zi'tah, checking as DC-VT to 75's, however their drop rate is improved that much. They will next you exp, seals, Moss, Cuttings and especially important, Wisteria Lumber. The lumber npc's for nearly 7k EACH. If you can 2box this, or farm with a friend this is an excellent place.
2) Sea Serpent Grotto Sahagins. This can be said for nearly all Beastment Capitals, but SSG is famous. You will need the Sahagin key to get past the door, but the treasure behind the door is worthwhile if you cna kill quickly and effectively. Behind the door you have Mindgazer's which drop the elusive Hakutaku Eyes that can be bazaared for 10-30k, Dragon Blood from the bats for 40k each, coral fragments from various sahagins for 8k. The main attraction here though is the BRD NM's behind the door. They drop nice maounts of gil, are mugable for roughly 3k and drop various scrolls that NPC from 500G to 4K Each. You'll also be receiving seals and exp while doing this so it works out nicely, however you MUST plan ahead for this. This is not a spot you go to farm when you only have 20Minutes, and not a spot to go when you only have 5 spaces open. If your able to have a second character sit in the zone while you kill monsters and have them near Norg, lot the npcable items and dump as you kill, it makes this much more profitable. Be prepared for links and death as well, once your behind the door you can't walk back through until you get another key.
*The HQ/NM Farm*
-This is the style where you look for the one item that's worth a nice chunk of change, or the NM drop. It takes time, and may not come until multiple, multiple, multiple tries. Make sure to check if this style works for your personality and if you're willing to have patience for a drop to come your way.
Some examples of this are:
1) Angel Skin in Kuftal Tunnel. On the basement map of Kuftal tunnel sits a small pond containing various monsters, one being the Devil Manta. This manta holds the elusive Angel Skin which nets you 400-800k depending on your server. It takes time to pull up with your fishing, and may not drop, but it's a nice chunk of change if it does. Bring along a darksteel ore during down time to pop Phantom Worm from the nearby ??? for an extra chance at Phantom Tathlum, as well as Yowie popping nearby.
2) Money-HNM's. These types of NM's are considered HNM, but aren;t too challenging. Serket, Bune, KA all fit this category as they have the potential to drop nice items gil-wise and are easily soloable-lowmanned by friends.
*The Consumable Approach*
-Every craft has item's that are used for many recipes that need to be restocked frequently. Become a supplier to this market and you're guaranteed constant profit. These items generally sell for less then some of your big-ticket items, but generally drop quickly.
Some examples of this are:
1) Silk Thread from Crawlers. Rolanberry Fields, Crawler's Nest, Maze of Shakarami, Pashlow Marshlands are all great spots with many crawlers. Thread is always in demands and generally nets you 10k profit per stack.
2) Beastment Blood from Poison leeches. The infamous Pashlow Marshlands is home to this farming spot. Located in one cluster are crawlers, bees, and leeches. All dropping items that are sought after by crafters. Utilize this spot for beginning your farming career to maintain income.
3) Spider Web from Spiders. An extremely fast seller, often selling before you get back to your mog house. It was once worth alot more before Aht Urghan came out. With Aht Urghan Attercrops right outside and a nice drop rate, these are worth 2-3k depending on your server.
*The Crafting Approach*
-This approach comes from your own crafts and skills. What did you learn as you leveled? Hopefully you didn't follow a guide solely, but took the initiative to look through recipes for other items to make. This will have given you a heads up on synths to steer towards away and to. Use this information now, go back to your roots. What did you synth on? What were you buying? See if the items you once bought are profitable to sell and fuel other's crafts.
Some examples of these are:
1) Fishing. If you can't make money with fishing you're doing it wrong. You can NPC fish for profit, as well as sell some on the AH for nice profits.
2) Goldsmithing. Use the guild to your advantage.
3)Woodworking. Shihei, the most common Ninja tool used can bring in profits.
4) Alchemy. Holy Water will never fail you and guarantee you profit without ever leaving the city's walls.