1. Know Your Limits
Too many beginners (and even more advanced players) try to do too much at once. As a teacher, I have had many students pick out a song or guitar solo that is unrealistic for their current abilities. Most students do realize the difficulty of their choice, but they are under the assumption that if they practice hard enough, they will be able to play it. Though I admire their ambition, I must explain to them that this is like trying to write a novel before you've learned to read. You can practice all you want, but it just won't make any sense and ultimately you will get frustrated and give up. Taking lessons from an experienced teacher is the best way to understand your limits and to learn to set goals that will gradually expand them.
2. Make Practicing Fun
This one sounds kind of obvious, but if you do not learn to enjoy the process of practicing you will be more likely to quit playing all together. In order to make practice fun you must first understand how and what you should be practicing. This will vary from player to player, but most beginners need to focus on 3 key points in their practice: coordinating the hands, developing the ears, playing enjoyable and rewarding music. A great teacher will design fun and, more importantly, relevant practice routines for developing new skills on the instrument. Learning to track your progress and implement frequent challenges for yourself will greatly increase your sense of enjoyment while practicing.
3. Play With Others
Playing music with other people is probably the number one goal for learning an instrument. However, most beginners see this as a distant goal that will happen someday after they have mastered the guitar. Actually it is extremely beneficial for beginners to start playing with other guitarists of the same ability as soon as possible. Seeing others go through the same process and overcome similar frustrations makes the learning process far less intimidating. Students in my guitar academy begin to play in small groups from the very beginning. The added sense of belonging to a community provides a valuable support network where students actually begin to help each other learn and stay motivated to practice.