Workshops are the reason a lot of people go to BJC. Even if all you do is provide a blank workshop timetable board, the jugglers will happily organise lots of good workshops by themselves. But you'll get a much better selection of workshops (and some much better promo material) if you put one person in charge of getting a reasonable amount of the schedule filled in beforehand. Workshop leaders might also give better workshops if they have had time to think about what they were going to do beforehand.
Most BJCs have had large noticeboard showing the workshop schedule where people just write things up when they feel like it.
You can ask on the pre-reg form if anyone is willing to do a workshop. Your Workshop Co-ordinator can also be shown the pre-reg data and call/email those people that they know will do workshops as well.
Some BJC have sheets where people can request workshops or offer to provide workshops that other people have requested.
Mainly, the workshop co-ordinator just gets out their address book and starts making phone calls and sending emails.
BJC 2010 rewarded workshop leaders with volunteer badges.
BJC 2008 wrote it into the contracts of all performers that they should do at least one workshop.
In BJC 2012, there were dedicated workshop "tracks" in different disciplines, and somebody from that discipline was roped into sourcing workshops. Jon Peat organised the ball workshops; Caspar and Ian from PassOut organised the passing workshops; Sean Morris did diabolo workshops and Mark Steyn helped with contact workshops. Each track had its own Google Calendar, and the more dedicated of the track organisers had access to the calendar so they could schedule the whole thing themselves.
Some simple web wrangling then allowed the resulting calendars to be published easily on the website well in advance of the convention.
One of the tricks involved in getting people to agree to run tracks, was to ask anyone who said "no thanks" if they could suggest some other more well connected people in the field who might be interested. Some people said they would not be able to schedule a whole track, but worked their contacts to create some interesting workshops, passing on details to the main workshop organiser. The overall approach resulted in a huge volume of workshops, which was only manageable because there were a lot of workshop rooms and because several of the track organisers did put in the leg work and schedule their whole track themselves.
2012 had a dedicated track for non-juggling workshops (mainly crafty), and a failed attempt at making a performance skills track.
As well as asking on a BJC facebook group for what workshops people would like and would like to lead, go to more prop focused forums like contactjuggling.org or diabolo.ca to find workshop ideas and leaders - this also helps to promote BJC. If you have people leading a track for that prop they might be encouraged to post to avoid duplication of work.
Kinds of workshops
Workshops don't always have to be juggling/circus based, very often other things can be popular with jugglers too. BJC 2008 had a good few workshops on aspects of performance... how to build an act, how to deal with artist contracts, and so forth. These seemed to go down well.
Either have different workshops for different levels in the main disciplines or make sure the workshops are accessible to different abilities.
Talented jugglers don't always make the best workshop leaders; it's worth deliberately sourcing some of the workshops from people with teaching experience.
Workshops don't always have to be in the same format! You can also have demonstrations, discussions, panel sessions, skill share sessions, presentations, or just "doing the same thing at the same time" sessions such as Stitch'N'Bitch or "let's all work on our five balls". You can source equipment that can be left in a dedicated area for people to play with. BJC 2010 had a unicycle trials course which was also used for a Parkour workshop.
Watch out for scheduling problems: if some of the rooms aren't available all the time, you need to mark this on your workshop board before people start filling it in. Other events during workshop time should also be marked on, for example, the business meeting or a midday closing show.