Go with nonmedicinal methods: a cool-mist humidifier and some saline nose drops to soften mucus, followed by gentle aspiration with a nose hose. For coughs, lots of fluids (nurse often, for short periods) can help loosen phlegm.
If your baby is 6 months or older and still suffering, offer an appropriate dose of decongestant based on her weight. Check the medicine's label or call and ask your pediatrician for guidelines.
Don't give your infant an antihistamine unless she is suffering from seasonal allergies (which are not that common in babies anyway).
Pass on multiple-symptom meds; if your baby has only a runny nose, she doesn't need a drug that tackles fever, too. (A fever may be a sign of something worse than a cold, so call the doc if one persists.)
Use a cough suppressant (look for dextromethorphan on the label) only if your baby is miserable and you're sure he doesn't have asthma (a clue: coughing mostly at night).