[Modigliani]

gotta give before you can take. so if u wan to CIM, u gotta make her really horny that she agrees to anything you ask for. work your tongue skills on her sensitive spots (ears, neck, around nipples) and especially a good lick plus plenty of teasing on her thigh and clit! Don't just "indulge" her, gotta deny occasionally, it makes it more fun for them, just like how girls cock tease guys =) Once shes high, you can start dirty talking and introducing the idea of all the wild things that is gonna turn both of you on so much (which includes seeing her finish you off with her sensuous lips and a CIM? haha)

of course, other various factors include clean shaved dick and hygiene. All my 3 ex and current one has allowed me to CIM and swallow mostly. Appreciation is important as well, acknowledge her effort. Don't just cum and then leave her hanging and forget about her needs too. "After sales customer care" right.. otherwise you won't get "returning customer" next time (returning favour).

Those are just personal experiences with my gfs though, sample size might be too small to generalise. There might just be girls who are absolutely against CIM. In that case, gotta respect her then. Bugging for it too much could annoy her and potentially hurt your relationship.