This was written a while back, and is just very basic and general, hope you enjoy. Some of this is opinionated and of course some may disagree:

I wrote this a few weeks ago and haven't posted it anywhere. Feel free to use this guide if you're in need, feel free to critique it and correct it if you see fit:

The intent of this article is to give a broad overview of the function of multiclassing. Caveat: For those that have an RP vision of their character with no regards to final product in terms of output (i.e. they don?t care how powerful they are, as long as they meet their RP vision), this article may not be for you. Why multiclass? In itself, multiclassing serves a few purposes: 1) It allows for greater potential in terms of overall power. 2) It allows for greater customization. 3) It?s something to do when you hit the cap. 4) It carries with it a greater potential to teach tactical combat, to the degree that it can exist within the current environment. Multiclassing gives you the following things that will transfer over to your other classes: Statistics, like Health Skills, like 1 hand slash. Some Abilities (I will define this later). Essentially, from an output based standpoint[/b], the only reason to multiclass is to gain in one or more of the above areas. There are Three basic types of character, or ?lines? of character, and knowing which classes fall into which bucket is important?not for what the class will give you, but for how the class will allow you to use your previous spells (and in some cases, even abilities). The three categories are as follows: A) Arcane B) Priest C) Other The main reason for categorizing the above list is for spell use. The vast majority of combatants in HZ use spells either as a primary form of combat or as support when the ability of choice isn?t up. The above list is bracketed such that for any class in an arcane school, they will meet the class requirement[/b] for all spells in that line. For instance, a Wizard can cast (this looks at just class definition and not skill points or level) the same set of spells that a mage can, or a sorcerer, or a conjurer, or a Chaos Warrior, or a Knight of Creation, etc. For the priest line, there is one exception, and that is the health line of spells, which are only useable by Clerics, Healers, Druids, Shaman, Spiritist, and Bloodmage. All of the priest classes, however, have access to the full spirit line, blight line, and nature lines of spells, such as Thunder, Thunder Cloud, ethereal paroxysm, etc. (Note that a cleric gets access to these offensive spells, plus use of all heals, plus it is the only class of the healers that can wear plate armor. This make a Cleric, in my opinion, one of the best tanks and best all around characters in the game). The above important for the following reason: Suppose Speedy Knuckleflash is a level 20 cleric and he wants to take up a second class to get some offensive power to go along with his character. What should he take? Well, if he takes something from the Arcane line (which he is not currently part of), he will not be able to use any of the spells as a CLERIC. (I.E. suppose he takes mage to 20. When he goes back to cleric, he will not be casting mage spells). This is not the case if he took Druid, for example. As a 20 druid, he would get access to various nature spells, and when he goes back to cleric, he?ll still be able to cast the nature spells he was casting as a druid. It is for this reason that in general (and I do mean in general) it is easiest to stick to one ?branch? of this tree. The exceptions to this rule are as follows: Cleric or Healer will benefit all classes because all classes can cast revitalize. Warrior is always a good thing to take because it covers a very wide range of needed things (good health, strength, good weapon skills, etc). Sticking to one branch when you start out, will offer a few advantages: 1) It?s not as harsh on the 100 spell limit. 2) Your classes will synergize better, and you?ll enjoy the benefits of multiclassing immediately. 3) It provides a semi-guide to allow you less to choose from. This is a good thing in some respects, because you?ll be more apt to make a good decision on your next class. For instance, suppose you?re 50 cleric/50 spiritist. I would recommend adding Druid before I would recommend adding 36 levels of mage for multicast. The overall output you?ll get from druid will be of much greater service to you than the multicast will, at this point in the game. (Druid will add nature abilitiy, which will allow you to use area attacks like crazy. This is of greater value than multicast at this stage of the game). However, suppose you?re 80 cleric/60 spiritist/60 druid. At this stage, adding mage for multicast, or warrior for melee abilities, makes perfect sense and is a logical choice. To the best of my knowledge, here is the categorical breakdown of classes. I am not certain on prestige monks but I am listing them to the best of my knowledge. Arcane: Mage Battlemage Wizard Sorcerer Conjurer Knight of Creation Chaos Warrior Flame Disciple Ice Disciple ? (Does this exist?) Priest: Cleric Healer Druid Shaman Spiritist Bloodmage Ranger Reaver Paladin Guardian Storm Disciple Spirit Disciple Other: Warrior Monk Berserker Crossbowman Spearman Scout Elemental Archer In Summary, if you goal is to create a toon that maximizes his or her skills and abilities, I would recommend sticking to the same tree for the majority (or certainly for the beginning) of your adventuring career.

EDIT: Of course, going outside your tree is necessary and certainly recommended, just realize that when multiclassing, the output decays as more classes are added. In other words, a rated 200 player stands less to gain by adding another class, assuming he built a strong build.