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.

Building a strong GPA is one of the best ways to be academically competitive. Most graduate schools of strong repute expect to see at least a 3.6 in relevant coursework. At that point, you may compete with other factors.

Time in industry does mean a lot, but it will need to be put forward properly. Make sure to show places where you excelled or where you were passionate. Where you were and what you did will certainly play a large roll in the significance of your field work.

In addition, experience with integrating computer systems will strongly contribute to being competitive. Proficiency in using either Raspberry Pi or Arduino to build systems which include sensors and which place data in the cloud will be very beneficial to any application in your field, academic or otherwise.