How to write an effective but very gentle reminder email to supervisor to check the Manuscript? I had submitted my first manuscript to my supervisor in 2013. He checked it after one year and finally submit it to a journal in Jan 2015. After one major revision, it got accepted with a minor revision and due date was 30th June, 2015. I modified and corrected the manuscript. But my supervisor was so busy that he could not make a final check and could not submit it in due time. I reminded and requested many times to check and submit it at the earliest in his free time, but my supervisor always says that "Sorry, I am busy and wait until my check." Editor had also reminded couple of times and finally closed the file. One day my supervisor told me that we need to withdraw this paper, because it is too late. I got shocked. So my supervisor might had felt bad for his delay, so he withdrew it and submitted to another journal in the same day of Mar, 2016 without any check. Fortunately the manuscript got accepted with a minor revision with a due date as 29th July, 2016. I corrected accordingly and sent it to my supervisor. But again seems same situation that yet he has not submitted.

During this period, I was ready with another manuscript. When I requested him to submit that, he advised me to submit by my own to XYZ journal as he is very busy. I felt very happy and submit it to his recommended journal in Mar 2016. This manuscripts came with a major revision after 2 months. So I revised the manuscript and submitted in due time by my own and informed to my supervisor. Fortunately, this manuscript also got accepted with a minor revision and due date was 28th Oct 2016. I modified everything and supposed to submit by my own. But my supervisor told that he will check at least once before final submission. So I could not submit it by my own and still waiting for my supervisor's check. 

So I am very much worried what to do in this situation. I have written and reminded many times. But as usual I always get a sorry reply. I am really worried how to write an effective but very gentle reminder email to him to check the Manuscript? I can't complain and be hard to break his faith on me.
I will just write how I approach the matter with very busy supervisors until now. It has worked without issues and our relationships has not been impacted at all.

The best thing is to take responsibility of the submission process (if you haven't done that already). When you have either the manuscript ready to submit or after you have done the corrections on the reviewers' comments, you can send an e-mail to your supervisor telling him/her that you have done all the corrections and answered to the reviewers (if applicable) and you are ready to submit. 

Put a deadline for the submission (not the journal deadline, but yours) and ask him to send his comments, if any, by then. You are planning to submit it on that day, unless you have his comments that require big changes. 

Also ask him if he needs to change the deadline, then he can propose a new one. But don't leave it without a deadline, as it could go forever.