Warning signs that would warrant a call to the doctor include:
* failure to produce enough wet diapers, or wetting fewer diapers than you know is normal
* any other signs of dehydration, including sunken-in fontanelle or dry mouth
* signs of struggling to breathe, including grunting when inhaling or exhaling
* chest or abdominal retractions, in other words, seeing the skin around the ribcage suck in past the bones with each breath
* blue lips
* unable to keep down several feedings in a row
My son came down with RSV at the age of 4 weeks. After he threw up three feedings in a row, I called the after-hours nurse hotline at my pediatrician's office (it was a Saturday night) and when I told them that he was also grunting when exhaling, they had me immediately take him to the ER. He was admitted and found to have not only RSV, but bacterial pneumonia. So I take breathing problems in infants very seriously these days. I don't mean to scare you, but if your mommy radar is telling you something is more seriously wrong than just a regular cold, please do call the doctor's office.
In the meantime, I agree with the suggestion to use saline spray before suctioning out the nose, and nursing as much as possible. You can also run a humidifier, especially in the room the baby sleeps in. Another thing we've tried with some success is turning on a hot shower and sitting with the baby in the closed bathroom, to give him a "steam treatment". A lot of times that really helps clear out the nose. Sleeping upright is also a good idea, if you can manage it somehow.
Good luck! I hope she feels better soon. There's nothing worse than watching your child have to endure an illness.