for me, it's more like when i'm not looking for someone. and that could be the case for me based on numerous reason, not just the fact that i'm with someone already. it could happen because i'm occupying myself with my work and keeping myself busy in a way where i'm not available. or, it might happen when i decide to isolate myself from others (it's the least likely of the other two scenarios) and not talk and communicate. when i do find myself communicating with others, i keep it simple. i recommend just making yourself busy though. having someone else in your life can take up a lot of your time so when you have that freedom to look after yourself, you should take the time to embrace that. it's been a while since i've been in a real serious relationship but i know it's difficult mostly because it's already hard to find balance when dating someone. sometimes you're worried about whether or not the dating will go anywhere, sometimes you're worried about whether or not she's actually interested in you or if you're just dealing with each other. and considering the fact i have a very persistent and committed nature, when i do find myself dating some girl, i try to make sure i'm doing all the right things to ensure that it will move past the dating stage into a relationship. but, now i realize it's better to let things take their course and let them occur naturally. that's how i feel with women now. at the end of the day, i end up dealing with more females than i want to, but it's better instead of putting all your eggs in one basket.
anyways, if you preoccupy yourself, you're always better off. do the things you want to do instead of worrying about "finding" someone. i swear, anytime i've found anyone worthwhile, it always happened when i wasn't trying to find them. the only women i've gotten through some sort of effort were the women i've dealt with for a week or two.
in regards to women redirecting their efforts when you're taken. it's natural because you're not preoccupied with them any more so of course they're going to exert more effort to get you so you refocus on them, it could be direct competition or indirect if they don't know you're with someone. either way, it doesn't matter. the point is you have to always be dealing with something regardless. when the time comes, they'll appear...