When a guy I like thinks I don't see it but he's checking me out. With a damn-I-want-a-piece-of-that look in his eyes. That makes me feel sexy. When I'm not attracted to him, it doesn't have that effect though, totally subjective.
Also when I'm just passing by, and they have a glance at me, turn back to whatever they're doing, and then uncounsciously look back to have a second better look. I love that! Depending on the situation, I'll act as if I didn't see it but have this huge grin on the inside, or I'll give him a wink.
This weekend I was at a club and this (total playerish!) guy came up to me and commented on how I was about the only woman in that club that actually knew how to dance. And how he loved watching me move. I knew he was a player, I knew it was a total lie because there were plenty women dancing, and I knew he probably said that line to about every woman he met there. Still made me feel sexy. (But then again that guy was totally on top of his game ^^) Before he walked up to me I had caught him looking my way several times throughout the evening. That "singling me out" made me feel sexy too.
Generally having the feeling that a guy choses me above all others. Whether you are at a club or in a relationship, doesn't matter, makes me feel sexy.
When I'm in a relationship, no matter how strong it is, reaffirmation of him chosing me, knowing he's thinking about me, knowing he wants me, will make me feel sexy.
It can be just a shared look between the two of us when we're in a crowd, talking to different people. It can be getting sexy lingerie knowing he's totally looking forward seeing me in it. It can be getting a text telling me he wants me and what he's going to do to me when he gets home.
It's really in a thousand different possible ways. But the message is the same.
Knowing he wants me.
I'm not saying that every little thing he does like that will make me jump his bones. It's the combined tension he's creating. It all adds up. Playing my mind like that, making me feel sexy, will get me in the mood.
When those "messages" stop, when that reaffirmation is gone, when I catch him looking at other women more than I catch him looking like that at me, I will start doubting myself in the relationship. -- I'm not saying he cannot look at other women. I can deal with that, if he's a man and something hot passes by he's going to look. That doesn't threaten me. Not as long as he's giving me those signals, shows me he wants me, and in a way, reaffirms that he's picking me over "them". He has to keep flirting with me AND he has to not take me for granted. (Which may sound silly but it's something I see a lot of couples doing when they're in a long term relationship...)
I hope that's what you meant by your question? If not, feel free to elaborate
