Used to sail two up quite a bit at night...we found it worked well if one person had a nap to about 9-10pm then we'd commence two hour about watches until morning. Two hours doesn't sound long but night watches can be boring and tiring and a two hour nap is usually enough to keep most people feeling fairly rested.
We'd always do a bit of a hand over when coming on/off watch i.e. what shipping is about, position, any navigational obstacles to look out for, how the boats travelling, sea state, wind conditions etc, reassuring to know all on board are on the same page.
If the weather builds and you think you need to change down do it early (some would says as soon as you think about it) rather then late.
Run minimal lighting on board except essential nav. lighting to preserve your night vision. We never used to read or watch movies as it trashes your night vision but did listen to ipod music and talking books pausing to do a 360 sweep every few minutes (it pays to listen as well as to look, you may hear something not quite right with the boat or in some cases hear another boat or obstruction before you are able to see it, one sailor I know heard the ship that almost ran him down in heavy rain before he was able to see it or detect it on radar!).
Be aware of shipping conditions, obviously in a heavy traffic area you need to be more vigilant.
Stow the boat so it's as quiet as possible for sleeping.
Set up a good sea berth, we used to pack extra cushions into the lee sheet berth or even put a cabin mattress onto the drop down in the saloon if that was most comfortable.
If your on watch and you find your drifting off stand up, dance a little, what ever it takes, there's nothing worse then being below off watch and hearing your crew mate snore!! Setting alarms is fine if your one up, two up and it just wakes the person trying to get some sleep and adds to their fatigue.
Enjoy, it's beautiful out there after dark away from it all.