Towel-resistance movements work best for me. Wrap a hand towel up place opposite of the side of the neck you wish to work, and pull back. Use neck muscles to push back against the pull unilt neck is straight. Repeat.
The most effective and practical method for training the neck is manual resistance. Have a partner put his hand on the back of your head and resist as you attempt to push your head back from the chin-to-chest position all the way back. Have him do the same side to side. Avoid going from the back to the front, as this has caused some pain in some people and is not really neccesary if you are already doing extension and lateral flexion. Neck machines exist but are somewhat rare. If you can find a safe one, try it. Hammer and Nautilus both make excellent 4-way neck machines.
On all neck exercises, make sure you use a slow tempo. Jerking the weight and sacraficing form for the amount of weight used would make you an imbecile. Don't do it...you're training your neck to prevent injuries, not to get injured! I would suggest one set of 10-15 reps for extension and lateral flexion.
To be a little more serious, try using towel excersizes. I do and it's made a difference. Ever play horsey with a 3 year old? Ever notice they like to move up and sit on your head? A strong neck is a must.
There should be a machine at ur local gym that you can use for this. I know we have one at our campus rec center so i would imagine it would be at any normal gym. It's hard to describe but it looks something like this. Not quite exactly but similar