I dunno lol, no tricks. Just 100% training, 0 range, good airbase, 10000 altitude, an air HQ, enough air support, make sure all the pilots except one are the same experience level and ignore for 3 months. come back and they should all be 50ish xp and 70ish skill.
For instance I just started a new campaign. All of my stateside and Hawaiian fighter groups I sent all the pilots to the pools except for the leader and pulled all fresh pilots into them. They're all mostly 35ish experience with the groups leader being 40-50ish xp. I'll ignore those groups until march and they should all have reached around 50xp and 70 airskill. They'll be in a range at that point from high 40's to low 50xp and high 60's to low 70's airskill. I usually at this point send them to the pools and recruit a fresh batch of trainees. I might just be seeing things but I find that if you have half a group of 50ish xp pilots and half with 30ish xp the high xp pilots stop training at all.
I keep a couple airgroups in use to train up the pilots who are stuck at 69 skill which can take another month or two of training (these pilots are the dullards I guess). These groups I just continually cull out pilots who pass 50xp and 70 skill and recruit not quite fully trained sub 70 skill pilots out of the pools as the months go by. I suppose that 1 skill point doesn't really matter, but I like the symmetry of considering 70skill pilots to be trained and ready and sub70 pilots to require further training. Makes knowing what sort of shape your pilot reserves are in easier too.
Navy bomber and patrol pilots take up a lot longer to train of course. It's far easier to just take them to 70 in a primary skill and then 60+ in secondaries then trying to reach 70 in multiple skills, but it still takes a long time.
This is why I dislike high xp low skill pilots. They take much longer to train skills then low xp pilots. I had real issues towards the end of the last game I played where my navy pools were giving me 40-50xp pilots.
Now... I might be understating the time it takes by a bit I guess. I wasn't this organized last playthrough at the start and it wasn't until later in the game that I worked out my training setup properly but I'm pretty sure it was roughly a 3 month turnover for training. If I wanted the entire group 50/70 + then it would take longer because there were always stragglers. Which is why I set aside 1 or 2 groups per training type to just train up high xp pilots who hadn't yet reached 70 skill. 4 months should easily see most of the newbie pilots in a group past the 50/70 mark, even if 3 months has a bunch of pilots still stuck at 47-49xp and 67-69skill. You're always going to get a handfull of pilots who pass 50 xp and all of a sudden can't get past 69 skill though, which is what I used the high xp training groups for.