I’m practicing the Harrier but I get a lot of wing rocking. Any tips to improve stability?

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make sure your CG is as far back as you are comfortable...

Sometimes there is a particular AOA that has the wing rock... try more or less AOA... find the sweet spot...

Create a mix for harriers... when the elevator reaches say, 75% "UP" deflection, both ailerons go up say 30 degrees... and opposite for down elevator... (Put the mix on a switch to disable for normal maneuvers)

Put a gyro on the Ailerons...

Get a differnet (Better) plane... just about anything AJ or EF should be awesome..
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Good tips above. I will add that a straight wing leading edge (like an Edge) typically fares better in harriers than swept leading edges. I don't like the looks of Edges, but I have flown a couple, and they definitely exceled in harrier flight, and I loved how they flew 3D. I think SFG's (side force generators) also help some.

Be sure to override the tendency to steer a little with ailerons while in harrier. I am bad about that and frequently fight wing-rock. Try to use ailerons only to keep the wings level, and steer only with the rudder.

my son's 61" Raven with the straight Leading Edge Wing harriers very sweet... upright and inverted!