The best approach to practicing is to make an effort to spend some time with your instrument every day. This could mean 5 minutes for some, and 3 hours for others. It is the consistency of your efforts that will yield the best results. Saving up all your practicing for an hour on one day of the week will achieve far less than 10 minutes every day.
For children, this is a fraction of the time they will typically spend in front of tv, computers, and video games. Finding 20 minutes every day to focus on developing a skill actually requires a very consistent commitment from a child or anyone else, but with gentle reminders from parents (threats, bribes, and coercion are usually counterproductive) it should possible to accomplish, and it will offer valuable rewards by developing the skills of concentration and discipline.
For adults, consistency can be very challenging. Adults tend to set rigorous or unrealistic goals for their learning and consequently they become very frustrated when they fail to achieve them. Focusing on the process and experience instead of the results would be very helpful for an adult with a busy schedule. Let your practice time be a meditative place where you can put your other concerns away for a few minutes and concentrate your mind on being involved in the experience of learning and playing.
If you are constantly beating yourself up about neglect of your instrument and lack of practice time, you’re doing a great deal of harm to your self-image. Remember that this is supposed to be something you do for enjoyment! If you miss a week’s worth of practice, just re-dedicate yourself and admit that you can’t do everything all of the time. Pace yourself and try to relax a little. Music should not add to your stress level.