Absolutely no idea. Bioware has never said anything about how they were going to go about doing that, but I'm sure there will be some way to meet with Liara or Kaiden or Ashley, provided Ashley or Kaidan survived Virmire, be it by comm or in person somewhere. The only other thing I can tell you is to be faithful and not pursue a romantic interest in the second game. If you wish, however, you can be completely unfaithful and pick a new partner. I wish I had more for you.