Trim, trim, trim ; and then trim again.
:D
More seriously, when anticipating an asymetrical loadout on the harrier (which does not like it at all), trim first, drop your weapon, then trim more.
Here's what I do, considering I'll drop a heavy bomb from the left side :
- start level and on autopilot (ensures a perfect trim)
- trim a bit to the left, up to the point I need stick input to the right to maintain level flight
- drop my bomb
- immediately trim more to the left, up to the point I'm more or less trimmed
- turn autopilot on again