A tip I have learned: keep your sitters alert. We had one seance where the energy dropped almost to a flat line. I later learned that some of the sitters were also trying their hand at trance or meditation and had zoned out both mentally and energetically.
After this, we always start with a video karaoke song vividly projected onto the wall outside of the seance room. We turn off the lights and sing and dance to the music right before going into the seance room. This has the effect of dramatically building the energy (a trick I learned when trying to teach novices to tune in to spirit for the first time). It also helps to keep the sitters more comfortable during the seance without stiffness from sitting still too long.
Some songs we’ve used: Walking on Sunshine, I Can See Clearly Now, Jeremiah Was a Bullfrog, The Road to Shamballa… Use whatever suits your taste as long as it is upbeat and peppy.