Diary/Log

Keep a diary/log of the amount of time you spend playing games. Since your perception of time is distorted during gameplay, it is hard to keep track of this duration in your memory.

Alarms

Set alarms that have 20-30 minute intervals. This will interrupt your flow during the game and you will have a constant reminder of how long you have played during this session. It is beneficial that you leave the console or computer at this time and take a short break. Eat a snack, start your laundry, clean up a bit around the house, etc.

Friends

Spend more time with friends that do not play as much games.

Other Activities

Find an activity that can be used as a healthy alternative, possibly one that involves face-to-face communication with others

Share your Problems

Speak to others that share or shared similar problems, they might be a source of inspiration

Time Limit

Allot a maximal limit to the amount of time you can play the game, either a daily or weekly limit. Check with your gameplay log to see if you are obliging to it.

Uninstall

Uninstall the game after a playing session. Usually after playing games for a long time, one feels guilty for wasting so much time. Use this momentum to uninstall the game - this will make it that much more of a hassle to start playing it again next time.

Parental Control

Ask a friend or relative to set a Parental Control on the game time. Many games/consoles/computers have this option.

Specific Time of the Day

Play only at a particular time of the day. This way, only a single time of the day will be associated with the gratifying feeling of playing games. This decreases the range of time that the cravings to play the game will be experienced.

Play Zone

Set a Play Zone. Similar to the previous point, limit your playing time to a single environment, for example, your bedroom. Do not play games in the same room that you do other important activities such as homework.  If you have more than one computer, restrict one computer for strictly homework/work activities.

Doing so will reduce the positive effects of the game, and gradually, your game time will decrease.