// assume we have globalVelocity and someParticle

float slowingFactor = 1; // change to make the fluid more viscous
if(sprite.xVelocity > globalXVelocity)
{
   // slow it down
   // since it's probably in flash, the time step is static, so this
   // gets called every frame, and every frame occurs at a set frequency
   sprite.xVelocity -= slowingFactor;

   // make sure we didn't slow it down too much
   if(sprite.xVelocity < globalXVelocity)
      sprite.xVelocity = globalXVelocity;
}
else if(sprite.xVelocity < globalXVelocity)
{
   // moving too slow, so speed it up
   sprite.xVelocity += slowingFactor;

   // and make sure it's not too fast
   if(sprite.xVelocity > globalXVelocity)
      sprite.xVelocity = globalXVelocity;
}