Setting up a Sleep Positive Environment
If you want your child to sleep well, make sure that your words match your actions. You can beg them to sleep until you are blue in the face, but if you are sending their brain cues to stay awake you are only shooting yourself in the foot. Think like a cave - cool, dark, quiet, and pay attention to timing.
Cool
Sleep experts agree, the optimal temperature for humans to sleep is between 60-68 degrees Farenheit. Since infants struggle to maintain their temperature, it is recommended to keep their room temperature between 68 - 72F. Follow the guidelines of their swaddle or sleep sack for how warmly to dress them for the temperature of their room. Here is Halo's and Merino Kid's guides to dressing an infant for sleep according to temperature. In addition to helping maintain warmth, a swaddle or sleep sack adds a bit of weight and confinement which settles the nervous system and promotes longer stretches of sleep.
Dark
The setting of the sun is a natural cue for our bodies to start winding down. We ruin this a bit by keeping our homes and streets brightly lit well into the night. In addition, our electronics have blue lights that shine all night long. Even just a small amount of blue light can trick our brains into staying awake. Cue your child's (and your own!) brain for sleep by dimming the lights in the evening and triggering blue light filters for screens. Any other electronics can have their lights covered with paper or tape. Keep the TV off in the evenings, as the light from that can cue your child's brain to wake.
If you need a light in the nursery to see for diaper changes, buy red spectrum light bulbs. Red spectrum light does not trigger brains to wake. If you live in a particularly brightly lit area or your curtains aren't doing the job at naps, consider installing black out shades.
Quiet
Although children can and should learn to sleep through the ambient noises of their living environment, sudden noises do startle. The sound of an ambulance, a parent dropping a pan, and an older sibling playing can all disturb sleep. Using a (non-repeating) white noise machine or turning on a fan creates an auditory focus point and drowns out other noises.
During the bedtime routine, quieting your voice and the sounds of the household, in addition to the cue of starting the sound machine will tell your child's brain it's time to move into sleep. If you must speak at night to an infant, do it in a whisper.
Quiet does not only refer to sounds, visual quietness is important too. A busy wallpaper or a brightly colored mobile can stimulate an infant. Move the mobile to the changing table to distract during changes, and think soothing when decorating the nursery.
Timing
Lastly, but most importantly, pay attention to timing. Young infants under 6 months of age don't follow the clock as much as they follow a pattern. Baby Sleep Site has an article with wake times by age. New parents often struggle in learning to read their infant's sleep cues, and missing those cues means their brains are triggered to stay awake longer. Once you miss the sleep window, it is much harder to get your child to go down, and the chemicals washing over their bran to keep them awake can affect sleep and waking all night long. Prolonged sleep deprivation can have lifelong health effects. Following the wake times on the chart makes it easy to guess when your infant will need to go back down. Children over 6-7 months of age can follow the clock. Have naps and bedtime happen at the same times everyday, and their brain will learn to wind down at those times.