What things do you consider when choosing a fixed base operator (FBO)?
I'll throw in my 2 cents here but I am by no means super experienced in this realm.


Amenities is not a huge deal for me. Generally I fly to go somewhere, while a nice interior is always a bonus, I'm fine as long as there is a bathroom and a receptionist to pay. Generally I'm at the airport to tie up and then leave and go about my business. Some place to sit to wait for weather to improve is nice but I by no means care if the couch is leather or 1970's velour as long as I can sit on it for a while. 
I flight plan with ForeFlight.com so I won't comment on what you should have but in today's day and age I would consider a WiFi connection in the FBO a must have since lots of people use tablet based planning now. 
Fuel cost is always a consideration and you should try and remain competitive to your local areas as much as possible. 
The key for me is a well laid out and clear website that lists prices for what ever you offer and other important information. Maybe an airport map with the location of the FBO clearly marked. A phone number to call for questions is also important.  
A solid relationship with a local taxi service is a bonus and a courtesy or rental car would be even better, but I understand how this could be a runaway cost on some level. 
Local knowledge is good if there are specific things that may be odd about the airspace and major airport issues that may arise.