Hi OP, I've been through this trauma photo thing a few times.
The first time I literally just forgot about it, popped outside the office for the pic when it was my turn, lived with the mediocre results.
Then I got wise to colleagues whose pics were lovely! They had made real efforts, including:
Salon appointment a day or two before for hair cut and colour
Salon appointment for a blow dry at 8am before work on photo day!
Salon appointment a few days before for semi-permanent eyelash extensions - ask the technician for a natural result, not too dramatic! They are really worth it and look great on camera
Wearing 2-3 times more makeup on photo day than usual - plus touch up with powder and lip colour right before pic
There are costs in doing this, of course, but IMHO it's worth it.
In terms of neckline, you are best off choosing whatever you think flatters your face best, I think that's quite individual? But usually at my office the only colours were black/navy/white - no one wore brights or patterns.
Many of the women at my office wore very carefully considered necklaces/earrings that were 'on brand' for them on photo day - I am allergic to most jewellery so didn't do this!
The other thing to consider is to be very charming to the photographer, smile nicely and be appreciative, be punctual and don't waste their time. Having said that, there is no harm in charmingly asking if you can try 3 different angles if they have time - 3/4 heads turned to both sides and then one straight on to the camera, that way you'll have options to pick the nicest one from.
Best of luck! :)