There is a real art to creating content that flies on Google+.
Every word, every image, each hashtag and every tweak in the formatting will make a difference how to people respond when they see it.

Let's begin with an overview of the basic principles, then I will give some extra tips too...

Share share and share

Thanks to Irfan Ahmad for the image, created as part of a joint project.

Comment on images:
Generally I would recommend ones that POW and WOW the senses - people respond emotionally to them and will tend to be drawn towards engaging more.
Test this for yourself, but also look at what other people are posting, and what is working for them.
Personally I know bright images tend to get more traction than darker ones, unless it is a starry night, of course.

Extra formatting tip:

When you share a new post yourself, there are four lines that will appear at the top of the post, before a 'read more' option - use this to your advantage:

Original share of GPlus Post
When someone reshares you post, or your reshare their's, you will see that only 2 lines remain visible.

Reshare of a GPlus post

As such, think about the formatting this way:
First 2 lines are always visible and therefore potentially influencing people's decision to engage
Second 2 lines are only an influence on the original, or as the new 'context' on the reshare.

I've been using this principle for a while now it makes a huge difference when you are choosing what to say. Enjoy!