It took me years to be able to swallow. I found it was much easier if I was still really turned on.
On the mornings that we have had sex the night before and I'm just giving him a "pick me up" blow job in the morning, and I'm not really heavily aroused, I still can't swallow. (I just hold it in my mouth and get it into a tissue he hands me.) But, if we're really into it and I'm horny I can.
It took me years, however. In fact, it wasn't until after he told me he liked to swallow my juices during oral sex that I was able to do it. I had no idea he was doing that, (being kind of.. busy at the moment) and I figured if he could do that, I could try to swallow his.