Clean, strong arms are imperative for me in hip-hop routines. Some dancers lack the technical training to understand correct arm placement. Try this: line the dancers up with their backs against the walls or mirrors, both arms against the wall at shoulder level and bent at a 90-degree angle. (You can also use elements and poses from your choreography that apply.) The goal is to increase muscle memory so they can nail the pose without the wall there. The wall helps with placement, preventing the dancers from having wild arms and moving beyond the pose.
Here’s another muscle-memory exercise that helps dancers remember to keep their arms engaged and working. Have the dancers hold their arms in front of them at shoulder level with fists clenched and shoulders pressed down, as if holding a beach ball. Press down on their arms, telling them to resist and to remember the sensation of resistance. This is most beneficial for popping and tutting and keeping hip-hop poses and movement tight, sharp, and powerful.