A few thoughts:
1) Practice makes it easier. Yep nothing beats practice, once you've done 10 shoots with 10 people you'll fast pick up your own methods, many of which might not even be things you really realise that you're doing. You'll also get better at prioritizing, controlling and going with the flow as you already have some idea of what is likely to take place.
2) Relax. Even if it means taking a breather/break part way through. Sometimes just a few moments to let things relax can really help out. It helps you just have a moment where you've not got to worry or focus on anything. IT might be it gives you time just to organise things and check what you have done and have yet to do and what might be needed etc... Plan for these breaks and build them into shoots, esp if they are longer events.
Like I said it doesn't have to be long gaps, but it is a moment to pause
3) Food and fatigue. Make sure you're well fed and rested before a shoot, a good meal (but not so large it leaves you lethargic) and rest can work wonders. Similarly fatigue will hurt your photography and make things all the more stressful and difficult.
Fatigue also goes further than just a good nights sleep; your pose, your shooting position, your camera etc... All these things can increase or lower your fatigue. If you've got a lot of shooting in one spot you might use a tripod - sure you can handhold for hours without issue, but a tripod just takes some of the strain away from you. That lowers the fatigue and gives you more energy to work with.
A few walks/runs/weights/exercise helps a lot too!
4) As ac12 said, write a plan if you're worried about forgetting things. Doesn't matter if its high detail or just notes - whatever works for you. It can help you remember to do certain poses, to watch for the light at certain times (eg if the sun is shifting angles and you want certain shots at times/places); it can even let you double check what settings/mode you need/use for a shot etc.... There is no shame in these - sure expeirenced pros of 30 years mostly don't need them, but unless you're doing this every day of a working week chances are you will forget things if you don't write them down AND consult those writings.
Heck I know that I have forgotten and rediscovered things, esp with editing, that I once did as standard practice. Getting lazy and skipping steps makes is so easy to then forget that that step ever took place.
5) And again, practice and experience. The more you do the easier it will all become.