This script will animate your avatar when attached and then loop the animation.  All you need to do is change the name of the animation:

// Basic Animation / Attachment 1.0
// Catherine Omega Heavy Industries
// Heavily modified by ArchTx Edo who is fully responsible for any stupid mistakes.

string gAnimName = "express_open_mouth"; // what animation to play?

default
{
state_entry()
{
llRequestPermissions(llGetOwner(), PERMISSION_TRIGGER_ANIMATION); // ask the owner for permission to trigger animations
llSetTimerEvent(0.5);//I added this line.
//llStartAnimation(gAnimName);

}

on_rez(integer param)
{
llResetScript(); // reset the script as soon as it starts.
}

attach(key id)
{
integer perm = llGetPermissions();

if (id != NULL_KEY) // make sure we're actually attached.
{ 
if (! (perm & PERMISSION_TRIGGER_ANIMATION)) // remember to use bitwise operators!
{
llRequestPermissions(llGetOwner(), PERMISSION_TRIGGER_ANIMATION); // request permissions from the owner.
} 
}
else
{
if (perm & PERMISSION_TRIGGER_ANIMATION)
{
llStopAnimation(gAnimName); // stop the animation

}
}
}
timer()
        {
        llStopAnimation(gAnimName);
        llStartAnimation(gAnimName);
        }
}