Experiment. I found night pumping dropped my supply, whatever books/experts said, because expressing is not like feeding and you need rest to get a lot of milk. I "power pumped" at an every 2 hour level for 2 days, followed by 3 hours the next day, and after that slept the night through - was amazed to see how much milk you can pump at the end of that. All the advice was not to sleep through the night, but you know what? I was the only person I knew who managed to keep an ample supply up for as long as my baby needed it, and I slept alternate nights through and always, always had a ton of milk the next morning after good sleep, so maybe try that and see how you get along. If it isn't working then you can stop, but the sleep has value in itself anyway.
When pumping, watch a DVD or telly programme that you really enjoy, and don't think too much about pumping. Relaxing helps and that works in doing so. Also I found breast compressions (kneading/massaging) really helpful in getting more milk out, and supply/demand meant more would then be produced. I also pumped until a few minutes after the milk ended, because that warned my boobs to make more. And work with what feels comfy; the settings don't need to be on the strongest if that isn't feeling good. Different women respond differently to pumps, as with anything else. Pumping straight after a nice hot bath when in a snuggly dressing gown was also helpful.
I exclusively expressed for my ds for 7 months (he had oral issues that meant he refused all solids till then) so he was on litres of milk a day, all extracted by me with an ameda lactaline. He was on the 98th centile, too.
I gave up following rules and just followed my body. It worked for me.