Based on your question, I'm assuming that you're talking about approaching guys at the gym, unless you're some kind of alien doing research on us.
I've always found offering a spot is a good way to meet guys at the gym. If you see a guy struggling with weight, then ask him if he needs a spot. This has always worked for me.
You can also stop and talk to them around the water fountain. I once had this hot guy stop me at the water fountain and compliment me saying, "man, you've gotten in great shape since you've started. What have you been doing?" I said thanks, and of course, I blushed, turned around and skipped away like a little school girl.