only add as much as you can handle and keep form. For instance if you can curl 12kg for 10 reps x 4 sets each arm, increase that to 13kg. Your reps might go down to 10-9-8-8 for the 4 sets. Keep that weight until you're back up to 4x10 and then increase the weight again. For larger muscles (like bench press or squats) you could increase more like 2-3kg each time.