If you feel uncomfortable just saying "Hi" for no reason, why not ask a question? As long as you turn it into a conversation, it should work out. Don't take this the wrong way, but it's pretty much the same "technique" many guys use in bars and such...use a question to start a conversation on the subject.

Honestly, there are few guys who are going to give a negative response, if any. Not only is it flattering, frankly most guys aren't used to it, so we don't have the "threat" response that women tend to have when randomly approached by men. If a girl came up and asked me about working out in the gym, a subject I enjoy (and most guys in a gym are interested in it, of course), I'm probably going to talk her ear off =). I would totally be into a girl interested in exercise and taking care of herself, not just because I do the same, but also because it's sadly relatively rare. Less soap operas, more tennis please! =)

Start off general, then ask them how they got into working out. Everyone has their own reasons, and you'll tend to start getting into their health, maybe their job, whatever. He'll probably start reciprocating the conversation fast, trying to find out more about YOU, if out of habit for no other reason.

Don't make it too obvious you're hitting on him in a creepy way, more of a "I'd like to know you better" way. You'll be fine!

The hardest part is getting over the initial start. I'll tell you what I've told guy friends of mine; practice! Don't think of each one as your "big chance." Go up, start a conversation with a guy, and try on another later, regardless. You might get a date, you might get a short conversation that doesn't go anywhere, but at the very least you'll get more used to and comfortable doing it. The more comfortable you look when talking to him, the more comfortable he'll be.

The funny part is if you don't approach each one like it's a big deal, it usually ends up going better, feeling more natural, and is less stress for both of you. It's not like you're going up to ask him out, you're just going up to say "Hi!" and find out a bit about him.

Good luck! Not many women would even consider attempting this. I predict you'll be very successful, but don't forget to be discerning! Hold to your standards and boundaries regardless of how it turns out, and you'll do great =).