pushups: do about 4 sets of 15 w/ only about 10-15 second rest and tell me if u still feel sore ... if u can't do full ones, then drop to ur knees to get them out ... then increase the reps and sets
situps: 1,000s of ways to do them, but here's 2 points: 1) if ur doing more than 30, then change it up to something harder 2) it's really hard to fatigure ur abs to the point of being sore the next day as they recover very quickly