A strong, defined pair of arms is hard to hide. Almost any form of clothing shows off your upper arm. President Obama's wife is constantly talked about in the mainstream about her defined arms. Here is a routine that will help you tone and develop a strong pair of guns!
Dumbbell Curls with palms facing down, 3 sets of 12-15 reps
Dumbbell Curls with palms facing in towards each other, 3 sets of 12-15 reps
Dumbbell Curls with palms facing up, 3 sets of 12-15 reps
The order of exercises are important because you are going from the hardest movement (when you have the most energy), to the easier (when you are more fatigued). Each movement is performed right after the other. When you complete the last curl, wait 2 minutes before completing the circuit again. The circuit should be performed 2-4 times based on your fitness levels.
Goodluck and be proud to bare arms!!