One of the simplest, and may I add one of the most safe ways to strengthen your neck is to use a towel. Simply roll up a towel, and loop it behind your head holding either end in each hand. Now use your hands to create resistance and use your neck muscles to pull your head backwards. You should do this from all four angles (behind, right side, left side, and from the front). You'll see an improvement if you do this consistantly.
I also recommend searching for 'neck harness' on google or yahoo shopping...personally my gym has a four-way neck machine which I find has less impact than using the harness and free-weight
lastly, you can also do the controversial bridge technique used by wrestlers and other athletes
