I provide accent modification services as part of my business in Japan, but only for working on developing an American English accent.
Here's what I can tell you about how it works for me:
If a client comes to me he or she needs to have functional English. If not, I refer them to a competent English teacher because I don't teach English.
I use a standardized and research based program with weekly sessions working on individual target phonemes determined initially by evaluation and culminating with reevaluation after 13 weeks. It is extremely structured and a serious commitment.
Between each session the client is responsible for daily practice and homework including listening and speaking practice. Listening practice includes auditory discrimination of minimal pairs, and speaking practice includes comparing past performance with current performance as well as me critiquing the recorded speech.
My clients who do well are the ones who put in a lot of time and effort. My clients who do poorly are the ones who show up weekly but don't follow through on the homework.
I firmly believe your best bet is to find a native speaker who is trained in accent modification. Then, follow the program rigorously.