How can I do alternate exercises between days for best results?

I lift three days a week—full-body workouts of three or four big compound lifts plus two isolation exercises. Doubtless, some of you lifters will take issue with that workout, but that’s not why I’m posting. I try to change the exercises up and not do the same workout two days in a row, but I always end up falling into a routine.

So here’s the deal. When I do, say, dumbbell presses each workout, I can increase either the reps or the weight each time. But suppose I replace it on one day with weighted push-ups, or dips, or military presses. Theoretically, these are the same muscle groups, but on the next day I come back to DB bench, I can’t lift any more than I could the last day I did that specific exercise. Thus, I progress in DB press more slowly than I would if I did that exercise alone. It’s clearly better to alternate exercises between workouts, so what should I do/expect? Shouldn’t my DB press be increasing even when I do a different exercise for the same muscle group? If not, I’m really increasing weight only half as often as I would if my workouts were static.