Learning to efficiently grind in world of warcraft can lead to better understand of the game, and, more importantly, more money and better experiance in a solo environment. Everyone grinds, whether we like it or not, so i will help you be able to make it more fun, and more efficient.

Firstly, what you grind is INCREDIBLY important. You should probably grind humanoids exclusively, they tend to have mobs that go down faster, and drop cloth for first aid and/or auctioning. There are a few exceptions to this rule though, if you are a skinner, you should grind beasts or dragonkin. Also, every now and then there are a few more exceptions, such as turtles in dustwallow(INCREDIBLE DROPS AND EXPERIANCE FOR THAT LEVEL!). In most situations though, humanoids are alot better to grind.

Secondly, know what you are grinding. Know where they spawn, what TYPES they are, what those different types USE (as far as abilities and spells), and most importantly : KNOW IF THERE IS A QUEST OR CHAIN OF QUESTS LEADING TO WHAT YOU PLAN TO GRIND! This may seem like common sense, but i assure you many people do not use a simple and fast tool called....THOTTBOT.COM... it will help you find mobs to grind, stuff that drops something you want, and quests!

Thirdly, while grinding, make use of your class to ITS FULLEST. Mana dumping may not always be the best choice for taking down a mob, try to kill mobs fast, but without losing much mana or health points, this will lead to shorter ammounts of time between the fights, allowing for more experiance per hour or drops per hour. Mages, make friends with mages and exploit them for their water early and often, every time you have to go grind, get food and water from a mage, in plentiful ammounts. This will assure you don't spend excess money on buying food or water. Another thing, bandages, while good, are not effective unless you have them created, do not use them unless you have them because cloth sells very well on most servers.

Fourthly, to deal with boredom, there are MANY solutions.
-Angle your computer in a position where you can watch tv and grind at the same time.
-Listen to music(lol, obvious?).
-Ventrillo and teamspeak with friends helps time pass VERY WELL.
-The phone can also help ALOT.
-This one will sound stupid, but simple homework(busy work, if you are a student) can also be done.
-This one may sound the wierdest, but if you know how to self hypnotise or meditate, it can give you the focus you need to do a long session of grinding(i have done this many times, thanks to a grade principle at my school who taught me to self hypnotise to keep my focus.

NOW FOR THE CLASS EXPLANATIONS!!!!

Warrior : Slow, high max damage weapon, with lots of attack power gear is IDEAL.
-Open with charge when you can get away with only pulling ONE MOB, more than one mob will slow you down because you will have to rest for a longer ammount of time. If the situation looks iffy for the pull, go ahead and pull with your ranged weapon, no reason to take a chance. ALWAYS execute whenever your mob is below 20%, until then, use your instant attacks(mortal strike if you got it is great!) for more sustained dps, and alot of burst damage. *** IF YOU HAVE ENOUGH MANA FOR A BATTLE SHOUT USE IT, YOU SHOULD HAVE BATTLE SHOUT UP ALMOST ALL THE TIME, IT INCREASES YOUR DPS ALOT***. At this point, you may need to rest, but if the mobs are green you should be able to just slice through them, in which case chain pulling is, indeed, awesome.

Priest - This one is a strange one to grind with, but can be very good in the right hands. Shadow spec is important, with, hopefully, improved power word shield. Lots of int/stam/spirit gear is good.
-Open with a self power word shield, pull with mindblast and cast shadow word pain. Mindflay while the mob is running at you, if you are facing a caster mob, you may need to silence at this point. Reshield if needed whenever. Mind blast as soon as the cooldown up, and when your not mind blasting, use mindflay.
***A SPECIAL NOTE ON SILENCE, NEVER SILENCE AT THE VERY VERY START OF A SPELL, USE IT WHEN YOU THINK ITS ABOUT 1/2 to 3/4 DONE, THIS WILL SAVE YOU FROM 1 OR 2 MELEE ATTACKS, WHICH IS AMAZING***

Druid - i could tell you how people believe druids cannot grind, but this is UNTRUE, i have played a druid the entire game, and i tell you, they do it more efficient than most. Important note, spirit is nice, but not mass spirit for your "grinding gear", try to use stamina/strength/intel. Feral combat is nice, but also not needed. (next patch it will freaking own though).
***The best part about druid is that it doesn't need rest 95% of the time, you can be in bear form and not take much damage, switch and heal, because you regen mana while in forms, keeping you at 100% mana almost all the time***
-As a druid you can choose to moonfire/faerie first(optimal), then flip to bear and spam maul. When the mob dies, you can switch to heal and rejuve and go to the next mob. VERY SIMPLE.

Mage - Very good, because you can make your own water/food and can avoid combat very easily. Ill explain how to grind as a fire mage, but you can easily apply it to frost.
-Open with fireball fireball, and continue to cast fireball until the mob reaches you, when it reaches you, frost nova and SIDESTEP(SIDE STEP IS FULL MOVE SPEED WHILE BACKSTEPPING IS NOT AS FAST) until max range, use fireball 1-2 more times, at this point it will be dead or close to dead and you can fireblast/scorch it to finish. Again, a very simple grind class.

Hunter - I love grinding as a hunter, pet's make it very easy to be entertained imo. The main thing is that it is very easy to forget to buy a full bag of arrows/bullets, please take extra special care in doing so!
-Probably the simplest of all classes, pets tank very well, and the dps is strong. So open with pet running in, aimedshot, concussive shot and autshoot it, serpent sting and rapid shot every time the cooldown is up. Mana is not a problem, usually the pet does not sustain much damage. IF YOUR PET TAKES DAMAGE, IT IS A VERY GOOD IDEA TO USE LOWER LEVEL BANDAGES ON HIM, IT CUTS DOWN ON TIME AND MANA.

Rogue - Rogues really do not have any trouble grinding, and i feel silly in even writing this. IMO a good, slow main hand is key to maximum damage verses mobs. You won't take much damage do to high dodge and very fast killing. Rogues can chain kill many mobs at a time without rest, i have done 15-20 mobs without even using a bandage on my rogue.
-As a rogue, it is a good idea to open with cheapshot. If there is 2 mobs, sap one and kill the other. Pretty much exclusively using Sinister Strike and Eviscerate with enough points. SIMPLE SIMPLE SIMPLE. i dont know how you can mess this up :O.

Warlock - One of the hardest for me to explain, because there is so many different ways of doing this. You can use either the soullink build or the affliction build, even a destruction build can be used. Whichever suits you, go for it.
-Some say, use dot's and whatnot. In my opinion, the best way to do this is to spam shadow bolts, and when you are low on mana you can mana drain a mob with mana or life drain a mob when your low hp. I like to use a succubus because they do decent damage, and when an add comes you can always seduce. Warlocks are very mana dependant, and you should take this into mind when preparing for a long grind.

Shaman - Shaman grind very well with a slow 2hander and windfury. Enhancement build is a very good choice for grinding, because you do not use much mana to kill the mob, and you can use your mana to heal yourself in between fights.
-Open with a lightning bolt pull, then melee to death. A good strategy i have found is to drop a manaspring totem, a strength of earth totem, and a searing totem, and pull every mob to your totem setup, that way you get max mana effictiveness of your totems, and don't redrop them for each mob.

Paladin - Very slow if you do not know what you are doing, but can be fast if you do.
-A seal of command build with a good two handed weapon is almost necassary, except for one exception i will explain in a sec. It is a good i deal to judge crusader and apply seal of command, this will increase your dps when your seal procs. You really won't take much damage, and when you are in trouble, shields freaking own.
-Alternate grinding strategy, very effective in mid 50's and grinding undead in epl/wpl. Go for a protection build(with an option of soc) and a good shield/1h weapon. Pull 3-5 undead mobs and apply holy shield, holyshield + redoubt will be significant dps on there own, BUT exorcism and holy wrath will occasionly significantly increase your dps. Reapply holyshield as necassary, and you should have no problem, alternate ideas would be to use shield spike and retribution aura, which will lead to more damage.