Things to consider:
Don't place the router next to an outside wall, interference with building materials.
Don't place the router next to any speakers, monitors, tvs or anything with a magnetic/electrical field.
Higher position produces better signal strength.
Use a gadget like Sirrus WiFi to see what channels your neighbors are using, and set the router to a different channel.
The default setting for a 2.4 Ghz radio is 130 Mbs and set for mixed b/g/n.
Use a metal shielded cable from the Modem to the Router.
Find the latest firmware for your router and flash using the instructions from the manufacturer.
Unplug/power down all components, initialize modem first, wait for all lights to stabilize.
Connect the modem to the router, power up the router, wait for all lights to stabilize.
Connect the computers ethernet cables and power them up.
If your router has a wifi protected setup (WPS) button, press, hold, release, when lights stabilize,
go to the management url in your browser, 192.168.0.1 or 192.168.1.1 and check/set your wireless
Make sure you have the latest drivers
for your wireless cards installed.