But is it always best to do "one more" or instead of doing "one more" increasing the weight ?
This depends on your goals . I found a great repetition scheme for people who has a goal but dont know how to achive it .
As you can see you have the chart . All you need to find suitable weights for you and do the targeted reps .
An example : You want to use 35 pound dumbbell at 8 rep . Next week do it with 9 rep . Other week 10 rep .
When you reach highest targeted rep get 40 pounds dumbbells and do 8 rep . And continue ... you get the idea .
-Have a great day.