No, you're not imagining it. Your doormen/supers/porters are, indeed, being extra friendly and helpful suddenly. They may not be rolling out a red carpet, but it's close (wonder why?).
Almost half of New Yorkers tip their building staff over $500 a year (total) as per last year's Brick survey. If you're not tipping at all, you're in the major minority. Also: maybe cheap?
Is the whole tipping process ethical? Should we do away with the practice altogether? New Yorkers are divided.
What tipping well will get you. Hint: window-washing and maybe even painting.
What tipping badly gets you. Hint: probably not the best service, but it can depend on your circumstances.
If you absolutely can't give any money (remember cash is, indeed, king), you may want to try cookies or wine (though make sure your doorman drinks). Gift cards are even better.
Please don't forget the porter. He may not be as in-your-face as the doorman, but he works hard and does a lot of the dirty work.
And, yes, your doormen are tracking your tips. Tip less this year than last or tip the night doorman more than the daytime doorman, and expect them to know.
Solving tricky tipping questions, like whether you tip less if you just moved in, how to divide tipping amongst roommates and more.
If you don't often see someone you have to tip, consider holding "tipping hours" in your apartment where the onus is on them to come to you. It's like college office hours. Only more expensive for you.