Demihumans may choose to multiclass at character creation. Use these rules.
Track experience for each class as individual totals. When you spend treasure for XP, the total is split evenly between each class you belong to.
e.g. As an Elven Thief/Magic-User, you earn 500 GP worth of treasure and decide to spend it all on XP upon returning to town. You would apply 250 XP to your Thief XP and 250 XP to your Magic-User XP.
A single class character will generally be a level ahead of multiclass characters. In the early game this could mean the difference between survival and death. In the later game it means quicker access to more powerful abilities, especially for spellcasters.
Hit Dice & Hit Points
At first level, use the largest HD between your classes to determine starting hit points. Each time you increase level in that class, roll for hit points as normal. Do not roll for hit points when you increase in level for the other class.
e.g. As a 1st level Elven Fighter/Magic-User, you use the Fighter's HD to determine hit points (1d8 vs. 1d4). When you reach 2nd level in Fighter, you would reroll hit points using 2d8. However, when you reached 2nd level in Magic-User, you would not reroll hit points.
Saving Throws & Attacks
A multiclass character always uses the best saving throws and attack values between their classes.
e.g. As an Elven Magic-User 5 / Thief 6, you would use the Thief saving throws and attack values.
Weapons & Armor
Multiclass follow the same rules outlined in the House Rules thread.
Can we sticky this thread, oh DM?