IMO your harness should not be too tight. That way if the harness rides up you just fly the kite low and wiggle your hips and harness slips down.
This is one solution, yes, but very individual I think.
I could never ride with a loose harness, as then it would rotate when riding waves when you get the pull from the side
I just HATE that - my hook should stay extremely fixed in order to work properly (thus a sliding hook is the worst I could ever wish...).
So I want the harness tight, so it does not slide around - and it will not ride up either this way.
Of course it can always move an inch or maybe two up and down, true but it will position itself low again, when your kite is low and you lean back - if you've had pull upwards to start with.
Actually, it does not have to be supertight, just okay tight and pleasant, in order to stay put
Others like a loose harness that can slide around, and you can "wiggle" down again after it has ridden up
And some like the harness hook to stay put almost totally fixed (like I do).
Apart from that, I agree 100% with what kiwimike writes here - go for a seat harness, as it gives many other advantages besides not riding up.
Later, if you for some reason want to change, you will know what to aim for and you have the technique (but you might have to test and use money on A WHOLE BUNCH of harnesses first, believe me - there is not one harness that suits everyone...)