Keeping in contact with professors after graduation? I just completed a CS undergraduate at University A (top 50 rank for CS), and am taking a gap year after graduating.  This past summer, I did an REU/summer research at University B (~top 15 rank for CS).  I hope to get recommendations from professors within both universities, and I have been thinking of asking if there are any opportunities to gain extra research experience within the 1.5 years of gap year that I will have, hopefully to gain another recommendation that can speak with regards to my research ability.

Has anyone kept in touch with professors you've known since graduating, and how do people generally do so?  I was thinking of asking one of the professors how I can best spend the gap year time that I have, but I was wondering if people maintain academic contacts like this after completing their undergraduate.
Yes, many people do this, and many professors appreciate it. Professors will often be interested in your successes, whether in academia or otherwise. Most will want to help you in your future, but if they have been reminded along the way then it is easier for them to comment and write letters. If you keep in contact with them they can also have better advice for you in questions such as you have.

It is especially useful to keep contact with anyone you considered a mentor and anyone you think is worth emulating in your own career. And a wide circle of contacts is useful for anyone, but especially for academics. Past professors can also put you in touch with people within their circle. 

If you have been away for a while, unlike this particular case, it is useful to make a campus visit and look up professors who you think served you well. Thank them for what they did for you. Afterwards it is then easier to keep a conversation going. 

But some very busy people and some others with whom you had very little contact might not respond. Don't take it personally.