I'm sure I'm NOT on your list of P38 pilots, but this is what I do. I auto climb out...hence no worries about trim. once I get to crusing alt, or if I level due to a "dot", I'll run auto level till I get my speed up. Once the speed is up I'll trim the plane for level flight at crusing speed. Level flight is usally aligning the red indicator with the horizontal part of the "L" in "ALT" on the trim gauge.
For most flying from here the trims will be ok. As I increase in speed, as in a dive, it will have a tendancy to try to go nose up, so as my speed increases I'll add some nose down trim to keep it trim out the way I'm use to. The trick is to remember to trim it back as you slow or you'll be fighting a nose low condition.
As I don't fly as often, and there by don't get the "practice" these "aces" get, I "cheat"
I have the key for combat trim mapped to a button on my stick. So flicking the CT on then off will trim the plane for what ever speed and attitude you are in when ya flick it. I also have elevator "up" trim mapped to my stick. It's great for when you dive after someone and they pull out of the dive on the deck and expect you to go "SPLAT", and you don't, and you got the big ol' P38 Screamin up their tail as the slowly loose there "E"
I'm still learning, but thats what I got