The most important factor is having a clear line of sight (LOS), otherwise the signal get's attenuated by trees, hills, structures, and gets bounced around degrading the signal especially if the tower is miles away. You could use one of those online cell-tower mapping sites, through I've found that they are often inaccurate due to crowd sourced data and changes in network management/topology. The most effective method I've found so far, is turning the directional antenna in a 360 degree circle (disregarding where the towers are shown on the map) in 10 degree increments while checking the signal strength/quality at each step, and often times you can find a better signal/speed via this method. Your mileage may vary.
Try the LTEFix store first, if they're out I would recommend a Proxicast or better yet a Poynting brand directional antenna. You need to spend between a 100-200 bucks to get a high quality, high directional flat panel antenna.