Practicing with MorseRunner is a good way to build up pileup skills in CW
The more you operate in contests, the more familiar you get with the common call signs, which helps your ears alot - if I hear DL1N I can start filtering on KS as likely last two letters, etc.
Similarly, though I'm sure it is different in KH6, knowing propagation helps filter out unlikely call signs and focus on coming back with more likely ones.
On CW, I try to lock in on the fastest one sending, since that will be the one that will stop first and keep the rate as high as possible. I use a 500 hz CW filter and also seem to lock in best on the lower audio.
I'm not a great SSB op, but I always try to come back NOT using phonetics, helps speed things up - if I have a partial copy, come back to "DL1 something, again" vs. Delta Lima One something" and when I finally get the call, just say "DL1NKS" vs. repeating his phonetics.
One other thing: with EU pileups, never give in to the rude ones. If I come back to "DL1 something" and IZ0ZZZ jumps in much louder, I will always keep coming back to DL1 something - if you show "fear" in an EU pileup, you will completely lose control!
Unless I'm operating someone else's station, I don't generate JA pileups like you must. But when I would op at W3LPL or remote op K4VV, JA pileups were the direct opposite - always come back ONLY with letters you are sure of. If you come back to JL1NKS and it was really JL1NKF on SSB or JL1NKI, you will be met with dead silence -until IZ0ZZZ throws his call in!
73 John K3TN