I try to do a couple of different things, some of which have already been mentioned:
Try to read up on available plot hooks and backgrounds on other characters to find out what other players are interested in working with. (Sometimes this backfires, because people don’t necessarily always want the RP they put in their plot hooks/backgrounds - especially things like police/detective backgrounds.)
Seek people out. Don’t be afraid to say “Hey, I saw X on your RP Hooks, or that you’re involved in Y organization–my character is interested in that,” and see where it goes.
Listen! Pay attention to what people are throwing out in scenes, and respond to it. Give them a chance to talk about the things that excite their characters, do things their characters are good at doing.
Ask them for help in their characters’ areas of specialty. Sure, you can PROBABLY do whatever it is by yourself with a little prep, but it’s so much more fun to say, “Hey, I could use a face/muscle for this thing I want to do and I hear you’re good at that.”
React. Don’t be determined to be “too cool for school” about other characters’ weirdnesses. They were built that way for a reason - be grossed out, scared, weirded out, whatever. Go ahead and show a little emotion about other characters’ schitcks.
Reciprocity. If someone asks you for help IC, or gives you a bone–do the same in return. People notice, and in my experience, they’re so excited to have a real back and forth.
