Practice does not come naturally and it just won't happen magically. Practice is a habit. In order to develop this habit, you need several things.
1. The Right Environment - Choose a place to play and stick to it. Have your music stand and music set up and ready. Make sure its not a distracting place like in the loungeroom or near the tv or computer! If packing and unpacking your instrument is a chore for you, purchase or make an instrument stand and keep your instrument out and ready. Make sure your instrument is always in a safe place, where it can't get damaged.
2. The Right Time - Try to play at the same time each day. This will help to reinforce the habit. In my opinion, mornings are best as it is when your brain is fresh and before the activities of the day start. As for how long to play, here's a recommendation for different stages:
* Beginners (preliminary to grade two) : Fifteen minutes per day for prelim. and grade one and twenty minutes for grade two, at least four to five times a week.
* Intermediate (grade three and grade four) : Half an hour per day, at least four to five times a week.
* Advanced (fifth grade and above) : One hour and upwards per day, five to six times a week.
This is a minimum guide and obviously if you do more practice than this, you will get even better!
3. The Right Attitude - If you think negatively about it, your practice will most likely be a negative experience for you. It's the same for all of us, you get out of something what you actually put into it. Have a positive plan. For example: "Today I would like to work on my A Major Scale and fix up the tricky bit in my new piece." Don't just play through everything without really thinking about it, as this can reinforce mistakes and is not productive.
4. The Right Encouragement - Parents, try to encourage rather than discourage. This can be a challenge but positive comments will always be better received than negative ones. Get involved. Have a family concert on Friday nights. Have a quiz. Listen to pieces. Ask Questions. And remember, there are a multitude of other musical experiences: live performances, tv/radio, online, cd/mp3 and all of these can help create a positive musical environment which will reinforce the habit of practice.
Remember that the journey is just as important as the destination, so try to enjoy the journey with all the little achievements along the way.