First of all, opening up your XAMPP to the whole Internet world can be very dangerous. You really have to get ALL the security set just right, including that of your router.
That said, what you want to do is really not a XAMPP problem, but a networking one. You need to set the machine running XAMPP to have a static address for your router. Just copy the IP and DNS fields shown when you run ipconfig /all in a command window. Now it will always have the same 168.192... address.
Secondly, you need to set up your router to pass the particular traffic you want to serve so that it goes to the XAMPP machine. This is done on a per-protocol basis from the router's "application" screen (or whatever they call it). Most of this stuff is router-dependent, and you need to consult their directions.
You'll also want to look at http://dyndns.com
to create a DNS name for those on the outside that will track your external IP address.
I have opened up only FTP like this, and i was amazed at the number of robots trying to crack my FTP account. FileZilla has some features to reduce this threat, and I highly recommend you use them.
I am doing a series of articles on my site about WAMPS in general, starting with XAMPP. I'll be adding a few on this networking topic, and another on FileZilla security as well. Check out bobstrand.com/totm and give me some feedback.