At each level I've either been adding a class that offers abilities that I really want, or because I feel the character just wants to enter that class (e.g. I decided at about 4th level that she was finally ready to become a Druid). If I don't have a clear choice, I'll pick a level at random.
Right now, going on 11th, I'd like to "go Rogue" to get the skill points, but going from 5th level to 6th level in Bard will get her the Suggestion bardic performance, which could be really useful, so I've decided to go Bard for the next level. Also, since she now qualifies for the Mystic Theurge PrC, I'll be wanting to pick that class up soon, perhaps within a level or two.
I guess it depends on whether I'd rather have the Rogue skills at 12th level, or want to get started as a Mystic Theurge.
I could have gone into Mystic Theurge for 10th level, but going from 3rd level to 4th level in Druid grants Wildshape, which is kind of an iconic ability for Druids. So now Nora can say that she is _really_ a Druid.
For 9th level, I also had taken Druid to get the 2nd level divine spells needed to qualify for the Mystic Theurge class. My memory is kind of vague about my choice selections before then, except that I had taken Bard and concentrated on that class because 1) I wanted the Bard spells; 2) I wanted to get to be able to cast 2nd level Bard spells to qualify for Mystic Theurge in the future; and 3) Since Bard was becoming my major class, I figured I'd keep at least half my class levels in Bard.
multiclass spellcasters, though, seem to have a major upper-level spell cost. I'm not sure which level cut-off is minimally problematic
In 3rd edition I had a cleric who, at 3rd level, took the Brew Potion feat because she was tired of being bugged for healing spells during combat. After brewing healing potions for everybody who wanted to pay her the manufacturing cost for them, I figured that she'd figure it would be neat to be able to make other kinds of potions.
So I decided to have her take a few levels of Wizard. This shocked one of the other players because her Intelligence was only 11. But I used my 4th level stat increase for Int and then she had a 12 Intelligence. Okay, she'll never be a very good Wizard.
My plan was to eventually increase her Int to 13 at 8th level, and take no more than five levels of Wizard, maybe six. If she took the rest of the levels in Cleric, then she'd be 15th/5th level at 20th level (assuming she ever got there) and able to cast 8th level Cleric spells.
But I was considering, especially since her Wisdom was only 14 (I had randomly rolled her stats and did not get very good rolls -- the DM even had offered to let me roll again), and given her personality, patron deity, and general goals in life, that entry into the Holy Liberator prestige class might be in her future.
Another multiclass spellcaster character I made was a sorceress/wizard/ranger in Iron Kingdoms. This was a 3.5 edition character, so she was getting raked over by the multiclass XP penalty rules, so I figured she'd essentially be like a Bard: a ranger who uses some arcane magic to complement her martial abilities. Eventually I had planned to go into the Adventuring Scholar prestige class, which is a bard-like character class.
Another Iron Kingdoms character I made was a necromancer/monk. Here, I had to contend both with multiclass penalty rules and the rule that monks could not gain new monk levels if they took any levels after becoming a monk. So, having established that she had one Necromancer level and then three levels of Monk, she was stuck with forever being a 1st level Necromancer and gaining Monk levels thereafter. But she had a pack full of scrolls, so if you needed someone to cast a first level spell and it was not a conjuration or an illusion spell, she could do it.