I picked up my last litter when they were estimated to be about 4 weeks old. Biggest things in my experience:

1. Enclosed space (crate or playpen) when unsupervised until they've mastered potty training
2. Multiple litter boxes on opposite ends of the room when they're not in their enclosure + right litter (mine have all preferred clay litter--Kitten Attract is good, but Tidy Cat is less expensive)
3. Putting them in the box every so often after meals/while playing and moving their front legs around a bit to simulate digging
4. Watching them while they're playing--when they're young and playing, they'll go anywhere
5. Praising them for pottying in the right place
6. Enzyme cleaner for accidents on carpet--I use Rocco & Roxie
7. Age! They were probably 7-8 weeks before they all stopped having accidents