The best way to prepare for an indoor show is to try to simulate the lines and courses you are going to see. If you ride in a big outdoor ring then try setting some jumps in a smaller area of the arena. At indoors, you often see a lot of bending and S lines, and the jumps come up very quickly out of the turns. Practice that. Your horse will also have a lot less time between the fences and has to make the same number of efforts over a shorter time allowed. You have to be able to think quickly but so does your horse, which is why I think you are preparing him well if you are able to simulate some lines and turns that you are likely to see at your first indoor.