I'm working on a couple of things and they both have subplots. The Riddick thing has a closed and limited setting so other characters aren't likely to just have stuff going on in their lives. Fewer opportunities for red herrings.
The other WiP is a Person of Interest Story. The setting is New York City and each character could have a dozen things going on at once.
Take Detective Carter for example. She could be working on something for Finch and Reese, working a case of her own, taking care of her son, pursuing a romance, searching for the perfect pair of shoes, wondering if the neighbors are going to play loud dub step until three in the morning again, trying to work in an appointment with her dentist, and worrying about the super coming to fix the garbage disposal when she's not there. Some of these could be subplots or red herrings others just distractions.
Right now I have three things that have been brought up and left to dangle, and I need to figure out what I'm doing with them.
1- Reese says he took the train to Manhattan at the beginning of the story. He lives in Manhattan, so that implies he spent the night away from home- doing what? He mentioned it mostly to mess with Finch's head and knowing that Finch will try to figure out where he went and why. So I have to figure out where he went and why.
2- Carter complains that she's in the middle of a case when Reese wants her to find out something. Either the case needs to be something just mentioned in passing or it needs to tie in either substantively or thematically. If substantively maybe the victim is the whoist and does that thing for the other whosit that is relates to the thing that the primary person of interest does.
3- The best friend of the primary person of interest has been attacked for reasons that have nothing to do with the plot except it ties into a theme of trust and caring. Of telling your secrets to somebody you think you are in love with and how they react to those secrets.