There's some good advice here. A few more ways to maximize the gold and levels you make:
1. After a certain point
enchanting begins to pay off very well. Most items in the game disenchant into substances that are more valuable than the item itself. Arcane dust, for example, is needed by more than one profession and you can make a fortune selling it on the Auction House. You don't have to try selling it for some ridiculous buyout, either. A reasonable buyout will bring you plenty of profit. It's a win for you because it will sell quickly, and you'll earn a reputation for yourself in the process.
2.
Grinding on humanoid mobs. Humanoids drop money, which is a good thing. They also usually drop some kind of cloth (the type depending on their level) which is even better. Use the cloth they drop yourself or sell it on the Auction House. Cloth is in demand by quite a few professions. You can also turn it in for reputation gains. If you're a dwarf and want to ride an Elekk (for example) then a good way to do that is grind for cloth, and turn it in to the Draenei Cloth quartermaster at the Exodar.
3.
Questing, and rest bonus. Pick up as many quests as you can in an area...and as a rule skip the group quests. Your most efficient leveling is done solo while rested. I generally only do group quests if they meet the following criteria: the quest reward is great, or one of the quest mobs has a chance of dropping something nice; it is part of a chain (or the end of a chain) that unlocks other quests for you, awards you rep with a faction, or attunes you for an instance. As a rule most of the group quests in the game represent an inefficient use of your game time, and waste of experience per hour. If you insist on doing group quests, the best way to accomplish that is to finish up all the solo quests in an area, then group up with others and work on all the group quests in one fell swoop.
4.
Only grind on mobs close to your level. Mobs that are green or yellow to you represent more experience for your effort than ones that are orange or red. The orange or red mobs take longer to kill, and will certainly cause more down time spent healing. The key to leveling is how you spend your time...the less time you spend healing and patching yourself up, the faster you will level.
5.
Fear is a Warlock's best friend. One superb method for a Lock to grind/level/farm is via
fear bombing. Fear bombing is a method whereby a Warlock is attacking 2 or mobs simultaneously with the assistance of the Voidwalker minion or, if you're lvl 50+ and fully specced in Demonology, the Felguard minion. Any spec of Warlock can fear bomb, though of course an Affliction spec will shine at it. You only want to attempt fear bombing in a moderately populated area. You also want to be careful that feared mobs don't bring more mobs back to you than you want. Ideally the mobs you fear bomb are non-aggro mobs...such as the Bloodpetal Lashers in Un'goro Crater...so other mobs won't attack you as they run around dotted and feared. How fear bombing works:
A) Send your minion to attack a mob.
B) Aggro another mob yourself by placing some DOTs (damage over time) on it. As it attacks you, Fear it.
C) Use Life Tap to refresh the mana you have been spending. This is the trickiest part of fear bombing to master IMO. You need to Life Tap often and then refresh your health by draining your victims.
D) DOT the mob your minion is attacking. Re-apply dots to your feared mob if needed. It *should* die as it is running around.
E) Aggro another mob. You'll dot this one and drain it...once your feared mob is dead, reapply dots and cast fear on this latest mob.
F) The mob your minion was on should be dead. If not, finish it off, and send your minion to attack a new one.
G) Don't forget to Life Tap throughout this process. If you time it correctly you'll do it during brief intervals and by doing that keep enough mana going to apply dots, use drains, cast fear, etc.
H) The process repeats itself. Health funnel (heal) your minion as needed. Use a healthstone if you need to. Be sure to drain the soul of your victims now and then to keep up a fresh stock of soul shards, so you can make plenty of healthstones (they come in handy). You can do that while your feared mob is running around dotted and dying.
If your minion is a Felguard they inflict excellent damage and can usually kill a mob by themselves without your help, except for you healing them via health funnel now and then.
Fear bombing is an art so don't expect to be a pro at it until you have had plenty of practice. It's good to learn how to do it early on so when you hit level 70 and need to farm mats for crafting (or just for selling on the AH), you can fear bomb an area and make good use of your game time.
6.
Paladin AOE grinding. Protection is in my opinion the best spec for a Paladin to level and farm with. I've spent considerable time running mine as all 3 specs (and combining them as well), and in regards to experience per hour and survivability, Protection is the best hands-down. The key to this spec is adds...the more mobs that are on you: the more damage you will do, the less damage you will take, and the more XP per hour you will earn. AOE grinding is an art form just like Warlock fear bombing, so it takes practice. You have to learn how to manage your health and mana as you grind. Seal of Light and Seal of Wisdom are your best friends, just as much as Holy Shield and Redoubt. Consecrate helps wear the mobs down. Seal of Light heals you, and Seal of Wisdom regens your mana.
Anyway, that's my contribution. I'm sure it's old news to some of you. I hope it helps the newer ones.