Other than not diving as aggressively in a P-38 as one would do in commonly known dive monsters (like P-47, FW190, F6 etc.), is there a way to prevent the controls from stiffening or is that an inherent trait of the P-38 when diving?
Thanks for putting the answer to this in English for us P-38 novices. 
Contrary to public opinion, you can dive the P-38 at high speeds safely. The P-38 dives best at medium to low altitudes, the thicker the air the faster you can dive the Lightning. Below 10,000ft, you can easily and safely reach speeds of 475mph IAS and slightly higher and retain control of the plane, although the controls will be sluggish due to the high speeds you're traveling at.
In a dive if I want to reach the maximum speed possible without suffering from the controls locking up, if in the altitude range of 20,000ft - 10,000ft, my dives tend to be low angle dives so I don't overspeed and avoid the high speed buffet and I will jockey the throttle to keep myself from getting too fast or to slow in the dive. I'll also skid my plane to keep my speed under control, those two rudders make pretty good air brakes and can save your arse in a dive if you get too fast. If you find yourself getting too fast and having a tough time pulling up, use the elevator trim to aid you in recovery.
10,000ft and below, it's balls to the wall in a dive and I'll dive at steep angles and use my roll rate to roll into the bandit as he tries to break to avoid my attack. In high speeds dives like this, I'll again use the throttle and rudders (skid the plane) to keep my speed under control and to avoid overshooting the bandit and giving him the shot, I'll go into a lag turn after my first pass if I miss.
