Get valid visa or AP.

It depends on which stage of process are you in. If you can get visa from US consulate, then you can go back and forth without any problems. The only problem is the visa you get will be 3-month/2-entry unless you are a Candian PR live close to the border(you might have a chance to get multiple-entry visa with longer period of time). Over this, you need to get a new visa again. Another option is Advanced Parole(AP) after you filed I485 for 6 months. However, you need to be careful in using it because Ap doesn't guarantee your entrance to US. Meaning, the IO at the airport can deny your entering US.