My junior and senior primary is not combined, but as a reverence question in general, this was a great question. I contributed some of my initial thoughts, but this is not something I could fully answer in a Facebook post so I wanted to share some of the things I do, and some of the things the primary presidency does to encourage reverence.
1. "You don't get a turn if you're not being reverent."
I usually use our "helping hands" jar to call children to the front of the room for an activity, but I warn them that if they are not being reverent, I will not call their name, even if I pull their name out of the jar.
2. I stop and fold my arms.
If the noise level gets too great because of a particularly exciting Singing Time activity, I stop and fold my arms and wait for them to notice and do the same. Then, I continue with the activity. This used to work a lot better when I was a new chorister and they did not know me as well, but it still works because the teachers pay attention, and draw their students' attention to my folded arms. Eventually, the room quiets down. If one particular child is not paying attention, I will gently call them out.
3. Enlist the help of the teachers.
I sometimes turn to a teacher and say, "I need someone to come to the front of the room. Brother so-and-so, can you choose someone that has been reverent today? I don't warn the children that I am going to do this in advance so it is nice to remind them that they are not being bribed to be reverent, so much as enjoying special privileges because they were reverent. Their teachers already know who has been reverent, and I can count on them to be aware of a child that might not be the most reverent in their class, but was trying better that day. I have other things I do but those are probably the three that happen the most frequently. All three work very well.
In addition, if the teachers are singing and participating, they are setting a good example for their children. I have encouraged teachers to participate in the silly actions before, as I really think the children love it and respond to having their teachers participate. They also help me to maintain reverence in the room, and if they don't, a few very rare and discreet times, I have specifically asked them to help.
4. Hotel bell
Our primary keeps a bell on the podium; one of those bells that you use at businesses to tell someone you need some service if there's nobody in the room. They rarely use it, but they always have it on the podium as a reminder. I have used it twice, ever. The key to making it effective is to rarely use it.
5. Point and copy
I have never done this personally, but our primary presidency will have the children copy them and touch their eyes, nose, etc., all with motions, to get the children to quiet down and show they are paying attention.
6. Keep a few tricks up your sleeves
I have some back-up activities to switch to or incorporate into other activities on days when things aren't going according to plan, from singing time activities that other choristers have shared over the years. These work better for junior primary, but I rarely have a problem with senior primary.
Things like: Singing Time glasses - I keep a pair of giant glasses in my bag. I can pull them out, put them on, and tell the children I am looking for a Super Singer to come to the front of the room, wear the glasses, and help me to pick out the next Super Singer.
7. End with a reverent song
I try to work on singing time activities that support the day's lesson as well as the song for the month. However, sometimes if I feel like the class is not properly attuned to the spirit to listen to the lesson, I will end with a reverent song, before turning the time over to the presidency member. I also try to close with my testimony, which is something our stake primary chorister recommended.
Last, I wanted to share what other Facebook post-ers said in response to the question that triggered this post: