Unfortunately, there are a variety of problems with regards to the butt, meaning there isn't one simple solution. After one particularly long ride I had several saddle sores, which are inflamed and painful abrasions caused by friction. To avoid saddle sores, there are a number of personal lubricants available to reduce the friction of long distance riding. My preferred brand is Body Glide. I don't use it for everyday commuting, but always for anything exceeding about 50 miles or so.
Clothing is another potential problem area.
Most bike-specific shorts are fabricated to avoid seams in troublesome areas. Riding with denim cut-off shorts or other heavy fabric may cause discomfort over a long haul. Occasionally, I also wear shorts with a padded lining for longer rides. A very important consideration is the type of bike you're riding. I wonder if your cruiser is the most appropriate bike for long distance riding.