Gosh, that's a huge question Richard. There are a myriad of approaches and methodologies depending on what you want to achieve (e.g. step change vs continuous improvement) and the context in which you're working (e.g. type of organisation, culture, etc.).

Overall I'd say pick a methodology, ensure it covers both the rational strategy and process elements of the change as well as the emotional leadership & change management aspects - and then stick with it. Persistence is perhaps the biggest key to effective change.

I'd also advise making sure you put in place some form of continuous iprovement cycle to make sure the change stick - anything based on the Shewhart Cycle/PDSA should work.