We had a kitchen/diner extension added in 2013. Our kitchen fitter was painstakingly slow - took over two weeks to fit everything...
I agree with others that you must be there to make decisions, particularly on the first day when so many things get set out and you need to know where you want everything to end up. If you're having wall units, you need to know how high you want them - I think a 50cm gap between worksurface and bottom of wall unit is normal, but we had to adjust ours due to a sloping roof on one side.
Worksurface height is another decision - 90cm from floor is normal, but you need to think about how level is your floor. Unfortunately, our fitter measured the 90cm perfectly in the corner where he started, but the floor is about 1.5cm higher by the sink - so our sink is only 88.5cm off the floor, which is irritatingly low.
Are you having new lights fitted too? If so, you need to be clear about where they should go to avoid casting shadows. One of our (accidental) successes was having one light either side of the sink, so we don't cast a shadow as we wash. One failure was the fitting of downlights so that they illuminate the top of the fridge more than the floor that we wanted to be lit!
If there are any 'temporary' measures you put in place, make sure fitters know where the final position is meant to be. Our telephone wire was plastered in to the wall because we had moved the phone while we had no electricity in one part of the room.... the phone is now stuck and can't be moved to the place we wanted it to be...
If you're getting walls plastered, make sure you know the dimensions that you need to end up with. Agree with PP about having kitchen designers on speed dial as our wall ended up a couple of cm too short and we had to get a cupboard rebuilt.
Put in more sockets than you can possibly imagine you'll ever need!
We bought our own sockets as we wanted particular ones. However, we didn't bargain on needing to have isolation switches for the cooker/cooker hood/dishwasher/fridge etc etc above the worktop - apparently regulations determine that many of these need to be easily accessible so can't be tucked in a cupboard. You will therefore need many more switches than you might realise!
Biggest thing is to keep your mind on the end prize - it's worth the aggravation to get there in the end. Good luck!