I use a lot of counters in my projects but always found working with bipolar incrementation tricky. This is probably not news to more seasoned forum members but I thought I'd share my method here. Feel really dumb that I didn't think of this sooner. Feel free to critique/improve:
This should work for any continuous negative counter (-1 to -LSB).
ldax increment ; any number from -1 to +1
AND $7fffff ; offset all negative increments to positive (GEZ) range (+1)
rdax counter, 1 ; add counter (negative value)
OR $800000 ; offset all positive values to negative range (-1)
wrax counter, 0 ; store new counter value