How to prepare for STEM graduate program while in industry?
If you haven't, reach out to your adviser and/or other professors at your undergraduate university for two questions. Can they give you any insights into why your application was unsuccessful? Do they have any suggestions? The second question could be expanded into--do they feel you have any particular weaknesses or strengths? What universities do they think would be a good fit for you? 

Possibly taking programming/computer science classes might be useful. If there's a university close by, see if you can take a grad-level or senior-level ME course as a non-matriculating student. If you plan to apply there, make sure that it won't adversely affect your curriculum if you end up going there.