My general rules of thumb for hosting along with finding rooms to race in are this:
1. Street cars on street tires, preferably SH or lower, usually offers the cleanest racing, although "cleanest" is a sliding scale, it's a relative thing. You'll find jerks in all kinds of rooms, just less in these. These rooms tend to attract a better, more courteous driver, and so when I host, these are the rooms I create.
2. Lower PP usually = better, cleaner racing. Most GT pilots are not skilled enough to drive cleanly with 500+ hp cars on any tire, racing or street. The faster the cars, the worse the racing usually.
3. Look for clear definitions in the room title and make clear descriptions when you create a room. The more specific the definition, the better the room. If it just says, "Racing", it generally won't be as well organized as "500PP SH, Clean". Avoid any and all rooms with any kind of stupidity or profanity in the room description when searching for a room, and if you use stupidity or profanity in the description you aren't very bright if you think it'll attract clean drivers.
4. Send FR's to good hosts, and good drivers. When you are about to open a room, go through your friends list and see who is online on GT and send them an invite message saying you are opening a room. If you get a good reputation, you'll get repeat business.
5. SRF off, when you create a room and when you search for one. Yes there are some good rooms with it on, but it's generally not used by more serious pilots, and tends to attract noobs and kids, who even if well intentioned, don't have the car control for good close, clean racing much of the time.