I was always taught to imagine the next chord ahead of where you are and use the chord your on to get there. Even if you imagine the wrong chord, as long as your playing continues convincingly you'll be ok. Think up a small theme and do several variations of it. Also watch for key changes in the chords because in such a case you may need to change the theme. That's my improv strategy, and it may be over-complicated or impractical but it's worked great for me. Think each chord as the music progresses and trying to build on that doesn't work for me. For some it does but I have seen many players who do that and it sounds like there playing in limbo. They can connect the chords brilliantly but there's no theme.

I think my views are very free-thinking and go more by creativity than anything else but I would like to hear where you see differently. Maybe even how I over complicated something and how you would do it so I may emend such faults.