Asystole is not a shockable rhythm, as the purpose of a shock is to actually stop abnormal electrical activity in the heart (as is the case in ventricular fibrillation) and allow the heart an opportunity to resume a normal rhythm on it's own after the shock's effects wear off. In other words, a shock also will produce a "flat line" but generally for just a few seconds (for more detail on exactly how a shock works, check out the blog entry here).
If using an Automated External Defibrillator (AED) the AED will announce "no shock advised, continue CPR" after it analyzes the rhythm.
In the ACLS algorithm, the only indicated drug is epinephrine 1mg every 3-5 minutes. The emphasis is on continuous high quality CPR, consider the H's and T's, and consider termination of efforts.
It is always a good idea to check a second lead for a patient with a presenting rhythm of asystole, because a second lead may reveal a fine ventricular fibrillation, which would present an opportunity for more varied drug therapy, and shocks.