- is it legit? (check code before adding for instamine, hidden premine, malware)
- does it have a real community? Do not add a coin that has no community
- avoid brand new coins to not get the blame for the scams (support established coins with longterm community like LTC, DOGE, UNO, RDD, FTC, ZET and so on because these coins are likely to support you too if you are legit)
- host never ico/ipo/ito and put a general ban on coins that were issued in that way with maybe only few well picked exceptions
- don't chase the hype to not be left with many dead coins on exchange for the sake of 3 days of volume (never go full bittrex)
- never delist a coin if you are the last market left for that coin
- never delist a new coin for low tradevolume too fast. If you add a coin you should keep it at least a few months no matter the volume.
- try to have good communication with the coin communities and devs to get things going faster for you
Just double and tripple check the coins are decent and people will like the exchange and maybe send the noobs your way
If you want to be extra high class: don't add coins with a premine (not even 0,1% for whatever excuse)
to sum it up: stick to established crypto that is proven to not be a scam and has real community because these are the coins that get you forward and you don't need to list, delist all the time and volume once attracted will stay with you if you manage to make otherwise good service (fast and reliable withdraw/deposit, good security, good support)