Again, think scientifically about what you're doing when it comes to improving your exercise performance, and in this case, the chin-up. While all of the suggestions above are bang on, think about the proper application of said exercises. Most people I see in the gym doing chin-ups or lat pulldowns hold their hand pronated (ie thumbs in, palms out) with a wide grip (and worst case, bar behind the neck). This is in the false hope that one's back will become "wider" compared to doing these exercises with a narrow grip. Is this logic sound? No. People forget range of motion and proper biomechanics (or the function of the muscles). Doing pulldowns and/or chin-ups with the hands supinated (palms in) and narrow grip (shoulder width) does a few things - but first, let's look at the primary functions of the biceps and the lat muscles - the biceps not only bends the arm, a lot of people forget that they also help rotate the forearm. Simple test - flex the biceps, put your hand on the muscle and rotate your hand. The biceps are fully flexed when the palm is supinated. Thus, by chinning with the palm in, you're maximizing the use of your biceps (some argue that the chin-up is one of the best bicep exercises). The function of the latissimus dorsi is to bring the upper arm to the centre of the body. By doing wide lat pull-downs, the upper arms are not fully in the upright position, thus limiting the range of motion of the lats.
Finally, when chinning or doing lat pulldowns, avoid the behind the neck method, as this puts a significant amount of stress on the shoulder girdle and may cause impingement.
Keep the positive and negative portions of the reps slow (2-3 seconds up, 2-3 seconds down) to eliminate momentum (makes the exercise harder - but is it supposed to be easy for the body to have to respond??) and reduce any chances of injuries. When chinning, begin the movement with a downward shrug by pulling down with the lats before smoothly transitioning to the arms moving down.
Stay focused, keep a record of your progress, rest, eat well and remember, you only grow stronger outside of the gym.