Depends on design really. My interior is a bunch of separately made items, all together forming one big U, and them mounted to eachother. Bulkhead becomes couch, becomes, closet, becomes kitchen, becomes cabinet, becomes bench, becomes other side of bulkhead. On their own any single freestanding piece would move a lot but all mounted together and you can shake the whole van from any part of it without so much as a wiggle.
As for individual items ... Totes, bungees, and shelf lips are your friends. I also like to use those rolls of shelf foam stuff since it helps hold things in place and cuts down on rattles and such. Heavy items like batteries and Aquatainers should be housed in a way which guarantees they cant move unless you want them to. Dedicated cabinets for them plus ratchet straps cover that issue just fine.
I know a lot of people opt for curtains but a sturdy bulkhead ensures nothing comes flying into the back of your head should you have an accident. I'll gladly trade the ability to have a swivel seat or plenty of room to recline in exchange for reduced risk of decapitation.
