Car won't start

I have a 1995 Nissan Sentra Gxe with 120,000 miles on it. The car will start sometimes if it is cold outside and the engine is cold as well. When it starts it sounds and runs smoothly. Otherwise I get nothing when I turn the key. The solenoid doesn't click, the starter doesn't turn over. All the lights, radio, and gauges still work. I've cleaned the battery cables and posts and removed the starter and had it tested. Both are working fine and I have no idea what else to check. Help please