Building strength in the arms is one of the benefits of the yoga practice. The strength is an essential vehicle to get to poses like wheel and astavrkasana. These three yoga poses are a great way start on the strength-building journey. Breathe five to seven full deep breaths in each pose before moving on to the next.
The photo above, warrior two, doesn't look like it's much work for the arms, and if you just kind of hang limply in it, it's not. However, there are subtleties of the pose that engage the arms, shoulders and upper back, making this my number one choice for toning up the arms. Keep the arms at shoulder height, fingers spread evenly, and fingers engaged. Arms should be strong and powerful, reaching out beyond the mat. Imagine that if someone pressed down on your arms they wouldn't move. Sink the hips low, push into the outer edge of the back foot.
Plank is excellent for core strength, but did you know it also works the shoulders and biceps? For this pose it's important to be on your toes and slightly forward (look at the tilt in my feet). Bring the forearms on the mat with elbows directly under the shoulders. You can be on the sides of the forearms as shown, with hands coming to touch. Engage the core, and feel a lift as the forearms activate and you push the mat away while keeping the butt in line with the body.
From plank, lift the butt up, drawing the bottoms of the feet toward the mat, and continuing to push the mat away from you with the forearms. Tailbone lifts to the sky, head is off the mat and the gaze is at the navel.