I want to get back in touch with professor so he won't forget me but don't know what to say! I took a class with him, but that's about it. I would like to keep in touch, so that he will remember me in the future but it's already been a few months, I just have no idea what to say, as I do not have anything to say really (Haven't been able to catch up with any research or so due to work). Any advice?
Had a very similar situation recently, with an academic I worked with over 15 years ago.
The email I sent was related to his current work (research displayed on his professional website - this is an important 'hook' I found), and also linking it to the research that we collaborated with all that time ago and the classes I took from him earlier still.
After the usual greetings, I firstly asked about about some of his current research that was of interest. Then in the next paragraph, I wrote a reminder of when we worked together in the context of how I have worked from there (once again linking it to his research). Then wished him well and signed off.
He remembered me - and answered my questions and we are now back in dialogue.