We had been trying to potty train Declan while keeping him in diapers for at least 6 months with little to no success. We used a variety of techniques I'll list below but ultimately what worked was to just switch him to underwear (and pullups at night) and in essence, let him learn from his mistakes. It's a bit messy but it only took 4 days and turned out to be much easier than I imagined.
Method... it's hard to say if all these are required: