While immersion chillers are great due to their simplicity and ease of cleaning, they tend to be a bit "slow" in terms of chilling speed compared to counterflow or plate chillers.  This is largely due to a lack of wort movement around the chiller leading to a cold boundary layer immediately around the chiller and hot wort everywhere else.  With a pump and a nice deflector plate it is possible to rig up a flow to keep the wort circulating, but for those who don't have the time or money, there is a pretty simple technique.

Most immersion chillers have a neck that bends out of the pot and then bends over to prevent any leaking water from entering the kettle.  If you are able to grab onto this neck (probably use a cloth to avoid burning yourself on the hot out tube) and simply rock the chiller on the side of the pot, you can break up the boundary layer and even create a little whirlpool with a little rotation action.

You don't even need to do it continuously.  You can give it a few rocks every so often and then get to some of your other brew chores.  As long as you break up that area and get a little motion, your heat exchanging efficiency will increase pretty significantly.  Sure, it won't be as great as some of the other chillers, but it can go a long way to saving you time and water.