You don't increase your weight / resistance on exercises.
If you are still using the same weights you did when you started or if it's been a long time since you increased, you're not challenging yourself enough. Our bodies adapt to the stresses we place upon them so if you mindlessly grab the same dumbbells or automatically set a machine at the same weight, you are not challenging your body to improve.
As long as you can still perform 8 reps with proper form, you're good. If you can't get 8 reps (unlikely if you added only 2 1/2 or 5 pounds), decrease the weight.
Notice I didn't say, "Increase the resistance when it gets easy". It should not get easy. The last 2-3 reps should always be challenging and when you can do 12, you need to make it a little harder by upping the resistance.
|If a baby can lift it, it's probably too light for you.|
Each workout, your goal should be to get 1 or 2 more reps (only with good form) or to increase the weight. This is called progressive resistance and it's important whether you are trying to shed fat, get stronger, or build muscle.
It makes no difference if you're using 3 pound dumbbells or 30. Follow these guidelines and you will see your results increase.