What is the best way to help a person who has difficulty changing position in bed?

To move them or to prevent bed sores. If for bed sores, get an air mattress. It's cheap. The whole setup is like $40. Problem solved.

If to move them, a pads work really well. As long as they are on a pad it's easy to drag them about. Sheets work too. A hospital bed helps position them towards or away from the headboard because you can put the head or leg portion up and let gravity do it's work.

Side bed rails are also helpful to give them something to hold onto while trying to turn or adjust self in bed.

Are you talking about repositioning to avoid bed sores or changes or sliding down the bed?

I would be sure and have a couple of extra pillows and a wedge to keep him in the position and to give you some assistance.

As stated Google videos and request PT and OT from his doctor with the understanding that you want to be trained in safe procedures for moving him. You don't want to wreck your back in the process.

I found that all of the therapists and aides were happy to show us how to move my sister.

Do you have a hospital bed? If not get your doctor to prescribe one. Be sure and get the one that has rotating air chambers and put a mattress topper on it, these help avoid bed sores.

Google hospital slide sheets & search youtube for some demos. Basically a synthetic (polyester?) small size sheet that really helps move someone around.

Use 2 big ones under whole body - 1 on top of other. Or can use 1 slide sheet by folding in half (open ends face direction you want to slide).
Tuck under person - person may need to be assisted to roll to each side of bed. Person will move easier. Take care not to slide off bed! Scrunch under small of back to remove.

I learned how to do the sheet roll thing for my husband. They showed me this one of the times he was in the hospital, You might want to google this.