I always have the gymnasts start their twisting passes w/ 2 or 3 layout halves, and then 2 or 3 fulls up onto 1 or 2 eight inch mats. Then they move to 1 and a halves, double fulls etc... that way they dont seem to get lost or i dont have to go back and reteach those halves....... I also spend a lot of time on the layout 1/2 at the beginning, it helps them really be aware of where they twist.
I know so many girls who can't do a half twist. I think it's important to keep referencing these lead ups. A half twist takes barely any effort once you know where to twist, it's just a slight shoulder drop and then you need to open up pretty quick. A full twist takes very little too, I mean you can full with very minimal arm pull once you know what you're doing. But again it's the open. I probably don't pull much harder for a 1.5 than I do for a full, I just open up earlier in the full.
For some gymnasts, for both the half and full, it may help them to think to open up and rebound out of the passes. But if they plan to compete them this can cause trouble eventually, so you'd need to weigh that. Telling the gymnast to open their arms up or out when they see the floor probably will help too.