You could use a free lunch every once in a while, right? Especially later in the month when you don't have as much money to spend. Here's a trick I've used successfully in the past, and I'm going to share it with you.

Take a co-worker out to eat early in the month when you do have some extra money to spend.

That way, later in the month, they'll likely feel obligated to invite you out. If they do, say you're broke and they'll offer to pay. (After all, you paid last time.)

Bonus tip: Take them someplace really inexpensive, like a fast food place or grocery store deli case. Maybe the returned favor will find you at a much nicer establishment.

This is one of my many schmoozing tips. The idea of schmoozing isn't a "You scratch my back; I scratch yours," but more of an "I'll scratch your back and expect nothing in return. However, if you'd like to scratch my back at a later date, I'd appreciate it." It might not work all the time and, sometimes, it might not work at all. But knowing you bought them cheap food will make you look like the good guy. Just don't get to the point where you're getting walked on.

I hope this works for you!