I read an article that suggested having a pull up bar in a doorway, and each time you pass through the doorway, you do some pull ups. The point was not to do the maximum you could perform (unless that was one, as it was in my case), but do short of maximum (maybe 70-80%). As I work from home and had a power rack in the room next to my office, this worked really well for me. And when I had to go to the office, I would try to get four or so "sets" in before I left, four more shortly after I returned and some more before going to bed. Because I'm a big believer in balancing push/pull, I would do an equal number of decline pushups. What was great about this method is that until you get to high numbers, it doesn't feel like a "workout" where you have to shower afterwards. I did this every day for months, until I started developing tendinitis in the elbows. It worked, but the tendinitis is something to watch out for.
After a couple of years of not lifting weights, I wanted to resume, including pull ups. I recently read an article that suggested if you can't perform four pull ups, you should focus on negatives. As I could only do two pull ups without kipping, I decided to try this method. What was key for me was to download an interval timer app, as it was way too easy to cheat, and set it for five seconds up and six seconds down, for a total of six reps per set. I add twenty pounds to a backpack, and it took a couple weeks before I could finish the second set. After a month and a half, I can easily do two sets of six pull ups (well, maybe I raise my knees a little for the last one or two). I did these as part of my regular back routine and I would recommend this method for anyone.