I'm big (6'2", 270 lbs (down from about 320)) and when I started training I could only fall hard and roll like a log. I thought I would never learn how to take ukemi!
Now I can take ukemi for black belts practicing koshi nage. There's no one magical secret. You just have to find the way (that's safe) and that works best for you - like most elements of Aikido.
The one key element for me was learning to relax my body (without flopping onto the mat like a wet rag) and learning to breathe in coordination with the ukemi.