Ok I know there are probably a Thousand guides on making plat but I figured I'd add my own personal experience from my past 6 years of getting coin because you can never have too much plat.
First off, if you have a life outside Everquest Job/Family/Kids which some of us do, the easiest and simplest way to getting plat is simply buying it offline from one of the farming websites, I'm not going to list any hear as per the forum rules but a quick Google search will get you going. I work in the real world to make real money which I feel gives me the right to trade it for in game cash, regardless of some EULA

. At the time of this writing I can find 100 plat for about $30 USD which in my opinion is a small price to pay for all the game time you gain instead of harvesting/farming and doing boring stuff since my time on EQ is limited by real life, this benefits me immensely. Keep in mind most of these sites are Chinese farmers and may take a few hours to get your plat together depending on your order, but I've never once been ripped off. Also what Ive been doing to prolong the life of my beloved plat is part two of the guide.
If you have a large stash of plat either legitimately or purchased coin you can take it and choose a few items on the broker to monopolize on. I choose the rare collection items. There are too many to list but any of the Expansion items that are in two collections, one of them labeled Kunark Collections; I'm not sure why but this shoots up the price depending on your server to 5 plat or more. I used the Orange Fishman Scale and Large Burynai Claw. On a weekday when the broker sales usually slow down (Again depending on server) after you've studied pricing trends for your chosen items, buy them when they are cheapest on the broker, buy as many as you can afford. The key is: buy only the ones under the price in which your going to resell them (obviously). This type of plat making takes dedication because you've got to keep an eye on your item a couple or more times a day and when someone puts one on the broker cheaper that yours buy it up and add to your stack (Keep in mind that you don't want to pay more for it because of Broker commissions than what you sell it for) Also get the Broker commission reducing items which will boost your sales and lower your prices without you losing any profits (This I know is a given in the game but really pays off here so its worth mentioning) High level players, big name guilds especially, with unlimited bank rolls will pay extreme prices for these items to quickly level up any twinked alts they have or new members, etc. I know I have.
Method 3: Before I had a job and a place in the real world that we all hate so much I used to spend countless nights just farming question marks hoping for rares to make some money. I found out from a friend of mine a long time ago where you can harvest the "cracked" bones for the collection quests. As always this can differ for prices on servers but on our PVP server Nagafen the rare cracked bones sell for way over a plat because again, the high levels wanting to power level alts for AA's and XP. The key I'm going to share is the location as to where these spawn and are almost guaranteed harvests. In The Commonlands, there is a graveyard just west of Crossroads at 129.05, -48.03, -204.26. Here you'll see the remains of 4 walls broken down with graves that have skeletons that spawn and sometimes one named in the Crypt in the middle. You can circle this area and a "?" will spawn about once every 3-5 minutes. Surprisingly the harvest rate for a rare cracked bone is (for me) about 33% on a good day. So if you do the math, thats roughly 4-10 plat every hour or so (If you get one every 15 minutes and they sell for a plat), of course its depending on your luck but this is the place I've found produces them most of the time. Also turn up your rendering distance so you can see them if they're far away. Also they're tricky because sometimes they'll spawn up against the walls and its all the same yellowish color so I use infravision in the game daytime hours which helps finding them. I'm not promising any hard numbers because everyone's servers are different I'm only sharing the amount of coin I've gained from personal experience.
Again I'm not a premium member so there is no way for me to find out if this has been mentioned on the site before so all I can do is give you guys the most detailed information I can and hope for compassion. I've read some of the other posts that I thought were good information but were shot down, which tells me this site is full of good information and again even if you guys don't give me the thumbs up I'll most likely buy a membership anyway so don't think I'm just sharing this information for a free ride, I in general just want to help out everyone. Also please don't bash me for buying plat, its just the way I play the game and I don't judge people so please give me the same respect. So please, let the flaming begin... lol.