What I did for my last campaign (Shadow World, all players having already played in the same world and timeline with me as a GM)
- lay out the ground rules about the general setting (centered in SW Emer but expect travels), the characters (custom RM, level 20 characters, no technical progression during the campaign), the way they'll work together (kind of cell for a secret organisation, I need at least two characters that are *members* of the organisation, including the cell leader, the others can either be members or "associates").
- let them discuss *together* the kind of character they want to play, both the rough demeanor outline and the technical aspects, and if they want to have bragging rights about a particular point (for instance, one of the players says he wants his character to provide the bulk of support magic - transportation, enhancements...). The GM can veto things if needed.
- take them separately to discuss the details of their character (race and origin, technical aspects)
- then, solo play (then duo play, then trio play) to roleplay major events in the characters' past and introduce a number of points of interest: people, places, items, knowledge.
- then, the four characters meet, and the campaign proper begins.
The pre-campaign (everything until the last part) took me almost one year. The campaign itself took five years to complete, at a rate of about 20 sessions per year (every fortnight or so) [each session was 6 to 8 hours long on average].
This is the kind of investment that is not necessarily worth it for shorter campaigns, and I will probably try to make it shorter next time, but I tend to fare better as a GM when I have spent some time with the players roleplaying part of their character's past - it gives me ideas, it anchors them in the setting and I can foreshadow the events that will be the core of the campaign plot.