Feed the baby
burp the baby (having her/him high on your shoulder helps to push out the air, I'm told)
wrap her/him tightly (not to the point of cutting off circulation) in a blanket
change the diaper (or get someone else to do it)
rock gently
get mom
give the baby a pacifier

If none of these work, the baby is probably crying for one of two reasons. Either you're evil or it's just time to cry. If the former is true, leave the baby in the arms of someone significantly less evil. If the latter is true, hold the baby and be nice until s/he falls asleep.