For me it was just challenging myself. On pushups, I just do (or try to do) one more than I did the last time. In my boot camp class a lady asked me the same question. I asked her to do one pushup. She did it and I told her the next time I saw her she would have to drop and give me 2. The next time I saw her, I said "drop and give me 2" and she gave me 10 instead!!! It's time, persistence and pushing yourself. Sometimes we don't know what we can do, until we push ourselves to do it.
I do this with weights too. If it feels too easy the last time, then the next time, I'll start with heavier weights. If I can't complete the set, I'll just switch to the lower weight.
On Wednesday at boot camp we had to do suicides with kettle bells. On the second pass, there were only 40#'s available. I told the instructor, I can't lift 80 pounds. He's said, "it's only for 1 minute. You can do it for a minute." So I picked them up and did it. I only wish I would have tried it the first time (for 2 minutes). Afterwards the instructor said "See what you can do when you push yourself."
I'm really learning to push myself with weight training and when I do, I see really great results.