Here are a couple of cues that can dramatically increase the results you see from both the chin-up and the pull-up:
1 – Chest to bar.
This will force you to extend your spine and anteriorly tilt your pelvis as you're extending your shoulder joints, which are also functions of the lats. So pulling your chest to the bar will cover three functions of the lats as opposed to one. When done correctly, pulling your chest to the bar will make your lats contract to the degree that they feel like they're about to cramp.
2 – Squeeze a tennis ball between your shoulder blades.
This cue will force you into retracting your scapula prior to initiating the pull-up. All three portions of the traps retract the shoulder blades.
If the above exercise looks familiar, it's because it is essentially the "sternum chin-up" which was invented by Vince Gironda. The Golden Age bodybuilders got a lot right when it comes to training.