This is a very good question with many different answers. I will vaguely touch on some of the different money making methods for members.If you would like more details on any one please feel free to contact me either in game or via yahoo. I do NOT know f2p moneymaking as it is a quite different game.
…1)Non combat skills
~Fishing~ Sharks, and monk fish, Fishing trawler for Manta ray & sea turtle.
~Hunting~ polar kebbits, carnivorous chinchompas (stackable)
~Farming~ Grow Marigolds, ranarr, snapdragon, spirit weed & toadflax.
~rune crafting~ is almost always profitable (You can kill Banshee's for noted pure essence a lot faster than mining for it ) There are worlds where you can run runes for others.
~Cooking~ Tuna Potatoes, curry stew, & pizza’s
~Thieving~ cave goblin wires from Dorgesh-kaan.
~construction~ Make Teletabs via Lectern in player owned house.
~Mining~ coal is slow but pays very well. If you are high enough Adamintite & Runite.
…2) Using Combat
~Dragons using Melee or Range.
Green dragons in the Wildy or in the chaos tunnels,
Blue dragons in Edgeville dungeon, in ogre enclave & there is one in legends guild.
Black dragons with 70+ ranged or Kill them with protect from Melee (use prayer saving armour for better profit)
(metal dragons drop unnoted ores therefore are not as profitable)
~Chaos druids, Unicorns, Moss giants, Vampires, White wolves, Cockroaches, Jungle spiders, Iban Disciples & Avanasis are all good monsters to fight for profitable drops.
*If you’re not interested in the prayer experience you can always kill and sell the bones. Ourg, Daganoth. Raurg, Fayrg & Dragon, sell best but even regular bones sell well.
** A fast way to big money is PKing but this is a risky endeavor I do not personally participate in.Remember there is always someone bigger and meaner than you... So you are liable to loose more money than you win.
…3) Slayer~ In general Slayer monsters pay very well. Consider the food you use an investment. Most assignments drops more than pay for the food needed to fight them. The possibility of receiving a rare drop (some worth up to 17m) makes them even more enticing.
…4) Merchanting...watch the prices on the exchange then buy popular items when low and sell when high. There are several items you can buy in shops all over Runescape then sell for profit at the exchange, Pineapples, seaweed, vials, bolts of cloth, Sacks, baskets, dyes, tinderboxes & Battlestaves are all examples.I DO NOT RECOMMEND MERCHING CLANS AS MOST OF THESE ARE SCAMMERS.
… 5) There are several quests & diary's that are well worth the effort.After doing the Throne of Miscellania (& Royal trouble) quests,You can get your kingdom workers to gather resources for you that can be sold. Ardougne and Sears village diarys give free pure essence and free flax daily.
…6) You can collect Snape grass, Mort myre fungus, Planks, White berries, Potato cacti, Flax,Red spider eggs, & Blue dragon scales. All these sell well in waves watch the G.E.
…7)Clue scrolls are a roll of the dice but they have the possibility of large large payoffs!
… 8) You can hang out at the Falador drop room... I find wearing a ring of wealth helps (this is controversial)... what you gets up to luck.
…9) Diversions and distractions... Penguin hunting can make as much as 104k a week & Shooting stars can give you 106 k a day in runes coins & ore.
...10) Try not to return to a bank with empty inventory & always pick up stackable items. Even if an items not worth much its worth more than an empty space in your pack. You can always drop an item if something worth more is available.
......The thing is the game is designed to take time (which is why my answer is so long).. Your suppose to have to work for it. Jagex does a good job of making sure there are no QUICK ways to fast cash! I answer this same question for a lot of people, often they want to know which one is the best method. There is no best method. They are all good solid ways to try out & figure which one YOU like the best (my personal favorite is chaos druids and slayer)... remember the point of the game is to have fun. So find the one you like, sit back relax and enjoy!