I'm assuming you are practicing to perform the Sirsasana and you want to strengen your arm muscles to make the head stand. Here are two exercises you can practice.
Planking. When you do the Sun Salutations you go to forward bend (exhale) into half forward bend, head up (inhale) and then float/jump/step back to chatura dandasana. Before going into the final pose of chatura dandasana, you have a straight body supported by straight arms. This is called the lower plank. From this lower plank you go into a higher plank (inhale) and back into the lower plank (exhale) practice this a couple of times. Ashtanga always does series of five of every pose. Kino McGregor has a nice video of this exercise.
Underarm downward dog. Go into the downward facing dog and then slowly bend your arms to get into a downward facing dog on your underarms. Fold your hands together and walk with your feet towards your arms until you feel pressure into your shoulders. Then move your head into your hands (exhale) and push up again (inhale). Do this exercise a couple of times. You can also get into the pose like Kino McGregor does it in her video.