Back in 2nd edition AD&D, I started using an option for multiclassing. It became pointless in 3e, so I had forgotten about it until the last month. You have the option of invoking this retroactively if you choose it before next session.
It was motivated by 2 observations.
1. 1st level in something gives you a bunch
2. Characters in literature and real life are rarely balanced in all their skills. They'll have a main domain plus a few other things they know.
From this, I came up with the sideline rules:
1. Each character, even if multiclass, can declare a sideline once any time in their career. They can choose any class available to their race.
2. The new class starts at -1000 or -1/10 of their current XP whichever is higher.
3. For now on, 10% of their XP goes to their sideline, 80% to the main class, 10% lost as overhead. (without the overhead it's too sweet of a deal.)
4. At -500 XP or thereabouts, the character gets one class ability as if they were first level, such as knowing one spell or one thief ability.
5. At 0 hitpoints, they get all of the abilities available to that class, including spells, weapons, and armor.
6. For hit bonus and all saves, use whichever class is best.
7. If the sideline is a fighter class, the character receives 1 hp for each fighter level. Otherwise no hitpoints are gained from a sideline.
8. The sideline yields no extra perks but the character can choose freely from perks for that class one the sideline is first level or higher.
This interacts in an interesting way with the XP tables, you don't get much bang until your sideline finally gets 1st level. From that point, the sideline advances at about the same rate, staying 3-4 levels behind. Once the main class hits slow linear advancement, the sideline is still in the double to advance stage and stagnates somewhat.
