Without a doubt weights, as heavy as you can manage - I see too many women doing 20-30 reps with 1-2kg weights which, let's be honest, isn't as effective as you may think. Try 3 sets of 10, 8, 6 of 8kgs+ each arm with various exercises and push yourself and make those arms work! In 5 weeks I have gained significant muscle mass in my arms doing just this. Don't be afraid to match the men!! Xx