For a general beasting:
French-Typewriter pull-ups in L-Hang, i.e. counting 3-5 seconds on right, 3-5 seconds middle and 3-5 seconds left (go back and forth 2-3 times or more for a set). At each side centralise your weight under that hand and try to take as much weight on that hand as possible i.e. 1 arm locking. Do at least 3 sets at different lock-offs, i.e. first set at 135 degrees, second at 90 degrees and third set at full lock.
For deep lock training:
I've found this very useful (it's very good if you've got short arms and have to maximise your reach!):
Pull to full lock for 1-2 seconds (visually mark the point on the wall in front of you with your eyes), pull deeper (mark the spot again) and pull deeper again). After a while, that 'visual mark' will get higher up the wall. It seems to be a good exercise but buggers the elbows if care isn't taken (speaking from experience).
Front lever cheating/progression tip:
If you're trying to progress to front levers and getting close (or just want to hold one for longer) try pinching the front of the pull-up bar with your thumbs- take's some of the strain off your back, making them easier to hold. Also using the same principle on a fingerboard: instead of using your thumbs utilise the palm of your hands to 'pinch' against the board on a decent (reasonably fat) sloper.
Core exercise (only applicable if you have friends and non-minging feet; maybe these to things are related?), a.k.a. Core Send-me:
From a vertical hanging position on the bar get a mate to hold out a hand and you have to reach up and touch it with your foot/feet. You go back to vertical and they move their hands and repeat etc. As always, maintaining good form and doing in control etc. etc. is essential!