There are a great many ways to achieve this, however, one thing to realize is:
Not all players will cooperate with this
For various reasons, some just won't. Some players will think this is mostly your job. Some players have serious difficulties designing characters up front-- if they put too much detail in before they are played, it no longer feels organic. Or put another way, some players not only dislike your pre-gen characters, they often dislike their own pre-gen characters. And hey, some players are just flat out too busy to do it.
Recognize these players and, basically, respect them.
That said, while there are many approaches, here are two I have had good success with:
Info packets and questionnaires
Unless you are playing in a well-established world or a completely wide open world, your players need some background information in order to be able fit themselves in to it. That's the info packet. Whether it's a few ages of text, or you talking at the for half an hour, or a Q&A session doesn't matter much. But give them something to work with.
Then give them an idea what you're after. This originally comes from the Amber DRPG, but I've used the general idea countless times in different settings: I will come up with 20 or 30 questions of varying specificity, and ask each player to answer at least 10 or 15 of them for his or her character:
Where did your character learn to magic/fight/cleric/steal stuff?
What do you love about your family?
What embarrasses you about your family?
Who was your greatest rival?
What is your ambition?
Etc. And do feel free to send the answers back for more detail if they write just, "Bob McRival" for the rival question, or similar. They don't have to write a book, but I do ask for a paragraph-- two or three sentences-- on each one. I used to incentivize this mechanically (a stat point, some minor starting treasure) but these days I tend not to.
Joint Character Creation Sessions
Whenever possible, I like players to create their characters together rather than just with me. I don't think of this as something fully collaborative like the chancel-building of Nobilis or the Covenant-building of Ars Magica, but I try to get the players to riff or build off each others' ideas for backgrounds as well as just combat synergies.
And I will explicitly and very overtly try to get each character to know at least one or two others (depending on the size of the group) and to get some rough back story idea of how and why that's so.
If I do all of those things with moderately cooperative players, I usually get more than enough material to work with.