For breaking it in, I wouldn't recommend soaking it. But you
can put hot water in a spray bottle, and spritz down the inside of the leathers where you want it to stretch a bit. Get it nicely
damp, not soaking wet. Then go ride til it's dry. On a warm day, that would be maybe an hour or two. Leather
will stretch when it's wet, and as long as the pressure that's stretching it is maintained, it will stay stretched. But if you get it wet, and then just lay it out loose, it could easily shrink. When they're dry, work some Neatsfoot oil in to the areas you wanted to stretch, and you should be good to go!
Also, regular use of Neatsfoot oil (Lexol) on the outside of the suit will help it stay supple and healthy.
And periodically washing your leathers is a good way to prolong their life,
if it's done properly. Salt in your sweat will actually eat away at leather over time. That's why it's so important to keep up on your horse tack... having a cinch or stirrup leather fail could suck if you were going over a jump.