Driving at a set speed on a motorway without changing lanes is a sure fire way to lose concentration. I really have respect for lorry drivers who can't easily change lane or speed up - and are speed restricted.
I do try and adjust my speed, change lanes (overtaking - not just changing lanes for no reason). I'm not suggesting speeding as in exceeding the limit, but I'm always more alert at speed on big open roads than driving slow where everything moves slowly (or appears to as the lanes are wider, signs are bigger etc). Try driving in Germany at 150mph and you'll be highly alert!
Using your mirrors a lot also helps, which you should be doing anyway, but I find that being totally aware of everyone around me - even when there's not really much need keeps you alert. Identify the vehicle, the speed, what it's likely to do. Look ahead and predict if someone may want to overtake, so you can pull out to allow them to do it. All things that keep you thinking.
Oh, and look out for the numpties in lane 2 that aren't concentrating on anything and are complete and utter zombies. They won't see you approaching or even anything ahead of them.
If the lack of concentration is leading to tiredness, get off the road as soon as you can. Park up and take a break or power nap - there's nothing else you can do. Fresh air and a coffee will keep you going for a little while, but not long enough to get to your next stop or destination.