How do I stop from heading up when not planning

[jingebritsen]

more mast foot pressure (MFP) will cease your rounding up.

arms straight, push down on the boom. those muscles worked: shoulders, lats and triceps.

[NOVAAN]

A 90 is a pretty small board. When not at speed it will tend to want to sink the tail and round up wind. Windsurfing is a balancing act with many things to consider. You must find the balance point for whatever speed your going.
In very lite winds You will need to stand on the center line of the board and the front back balance point for you and the board. I sometime need to have my front foot in front of the mast on my small board. At low speeds the board will not react to foot steering the way it does at speed. Often its opposite of what you think. 95% of steering at low speed is done with the sail not the board. You need to have quick feet to react to the balance and saol pressure...

[NOVAAN]

sail pressure

[dllee]

When planing, you are back on the board, or in the footstraps.
As soon as you board comes off a plane, step forward to slogging position.