I started my career as an engineer and ended up managing engineering teams for the last 5 years. Ability to hire/manage people and communication skills are very valuable skills because as an entrepreneur you end up hiring help sooner or later.
The best advice I can give to an engineer working on communication skills is - learn how to adjust your communication for different audience (biz, techies, artists, foreigners). The key is practice of course. A good trick with written communications (email, blog posts, online projects for freelancers, etc) is just to not send/post as you finish typing - save as a draft and come back to re-read it, if you can shorten it or make clear, great.
A very close 2nd is sales skills, which comes down to ability to ask for things and building tolerance for hearing no (simple, right?). Here is how you start - in your everyday life start asking people for things - better table at a restaurant, room upgrade at a hotel, discount for your purchase at a store checkout. You will be surprised how often you will get things for asking, and hearing no doesn't set you back in any way. When you run your business you will be asking many people for many things - help you spread the word, sign up, upgrade, etc.
My two super powers (oh okay, soft skills) are:
1) The ability to read people quickly and manipulate them accordingly. Admittedly not as well as Kevin Spacey's character in House of Cards, but I'm still working on that.
If you want to practice this, you should read up on empathy skills and always think from the perspective of the person you're talking with.
2) The ability to think logically and critically at a fast pace. The down side of this is that I have to listen to you finishing your story, that I've already figured out and a response and a response to your response.
No idea how you can practice this, except perhaps reading up on critical thinking.
The downside to revealing these soft skills is that I come across as arrogant, when in fact that's not a word people use to describe me.