We all get jealous here and there but when you stop and think, remove yourself from the emotion and look at it from a distance you find it's often silly.
Here is how I've learned to view this:
Just because I have someone that I think is beautiful doesn't mean that my eyes won't wander. I can appreciate beauty in other women but it is going to be my woman that I'm going home with. That's really all that matters. Now, if I'm getting another lady's phone number that's when its an issue, and I expect it to be. Talking to her isn't a big deal at all. If I'm talking to her and touching her without a medical reason that's when it is a problem.
The women I date I give the same freedom. I'm not their father, and they're not my mother. If they're going to cheat, guess what? I can do absolutely nothing to change or prevent it, control is completely an illusion. Cheaters are going to cheat, players are going to play and haters are going to hate. And if it happens this hater is going to walk.
This is where trust is the foundation of a relationship. In the beginning I make it clear that I'm not going to dictate what she does and won't be dictated to. The best preventative is not putting yourself in that situation to begin with. If it happens, I'll show mercy once and none the next and expect the same. It's a respect thing. It's fair.
Don't take it too hard your guy checks out other women. He really shouldn't do it in front of you but he's going to do it and the quicker you get over it the happier you'll be. Don't forget, he's going home with you, not her. Anything aside from that is your own insecurity. It will eat you alive if you let it continue. How do I know? I've been there ... learning to let go is something you learn, not something you're born with. There is no jealousy in true love.
A lot of what you're seeing could also be your imagination. I don't 'subtly' gaze at anyone. If I'm looking openly it's not hard to figure out. If I'm not looking openly, you'll probably never catch me looking. If a guy is checking out a girl his eyes are going to linger at one spot or another. Most likely her ass or her breasts. I like eyes more than breasts though.
Yes, I still get jealous but I dismiss it quickly unless red flags are starting to pop up. In that case, communicate but never accuse. Communication is the doorway to the future. Accusations is the doorway to the past. Altercation is the doorway with bars. :)
As long as your guy is happy at home you probably don't have anything to worry over. Aside from those serial cheaters out there we tend to stay where we are happy, much like anyone else. Eyes wandering can be a red flag, but only if you know he isn't happy. In that case you need communication to fix that.
Dressing slutty like they do *can* help (maybe at home around the house when you're looking for a little seduction) but don't change how you are, there's a reason he's with you. Personally if my woman is going to be a slut I prefer it in the bedroom; I like women with class and value. If you dress that way on the street he's probably still going to look at other women and if he notices you're competing he may subconsciously use that against you, or consciously if he's that type. With some guys its a reflex more than an action. It actually can be innocent.
If it makes you feel bad take to looking at other guys. If he makes a big deal about it then it's a good time to have a mature talk. I'm not saying make it obvious, that's a bad idea, I'm just saying feel free.