I was blessed with an "easy" first born who turned 3 in February. Aside from teething and sickness, he's been a good sleeper. When we welcomed his little sister into our home this past September we were in for a rude awakening because (1) she was colicky, and (2) our oldest started throwing tantrums before bed each night. So, baby crying + toddler crying = mommy crying.
My grandma suggested we establish a new bedtime routine to give Will some much needed attention. The baby was going to cry no matter what, so I would set her in a safe place and snuggle with Will to read a bedtime story. Worked like a charm. This new routine has greatly reduced the number of tantrums at our house. Grandmas know all.
Now, Will still likes to get up at night and either crawl into bed with us or start making a mess with water in the bathroom. Haven't fixed that one yet.