No tell her beforehand, she will resent you if you tell her after. To a woman this feels like you stole something from her and she might fucking hate you for it. So don't sleep with her again under false pretenses.
Meet up with her and give it to her straight, put a focus on 'having fun' rather than 'having sex', like say stuff like 'I really enjoy being around you, you're a lot of fun' rather than say... 'You're awesome in bed, we should do it again soon'
The best way to go into any sexual relationship with a girl is with the mutual understanding that the two of you are friends with benefits. i.e. Friendship comes before sex but only after you have already had sex? Do you know what I mean? You will have healthier longer lasting sexual relationships with these women if they feel like there is more to it than sex.
Once you have had sex with a girl once she is soo much more likely to have sex with you again unless you were terrible in bed or something, so if you are the kind of guy who likes having sex with multiple women then you want to have lots of girl friends who you have already had sex with but don't expect a monogamous relationship from you.
Here read this article "10 Tips for dating multiple women"
http://wayoftheplayer.com/?p=993 it will better illustrate my point.
When you're away, you are going to meet a lot of girls who you cant have relationships with so they already don't have that expectation of you because you both have an understanding, the relationship is near impossible right because you live on opposite sides of the globe? But that doesn't mean you can't have a good time.
You can do the same thing with girls that you hook up with in your home town, they just need to understand that a relationship is impossible or very unlikely because of who you are and what you want out of life but that doesn't mean they wont still fool around with you, in fact for a lot of girls it is a relief to find a guy who isn't trying to rush things and know's how to play it cool.