I think it depends on who you romanced. If it was Ashley or Kaidan, you get the options pretty early in game to tell them you're just not interested anymore. Garrus and Tali only work if you began their romance in ME2, as do all other ME2 romances. Liara, I know you can start again with her regardless of your choices in ME1 or ME2.
Each romancable character will send you an email at some point asking you to meet them somewhere (Citadel lunch/date for Kaidan, Ashley, and I think Liara, otherwise it's just on the Normandy unless you have a ME2 relationship) and you'll have a conversation which brings up the option of being "more than friends" (in fact, I believe it's even paraphrased as that on the wheel).
It's pretty much first come, first serve, so choose the scene with your romance before any other characters to avoid issues (as the scenes can overlap). And don't worry, the options are fairly obvious, and won't trigger anything if you've already locked in one specific romance over another.
Edit: As for bouncing back and forth, I don't think that's an option. Like I said, locking in one romance cuts off all other attempts (at least, it did for me).
Modifié par LadyofRivendell, 18 mars 2012 - 04:05 .