Where the (-) cable attaches to the frame (or engine), clean the cable and metal surfaces.
Also where the (+) cable attaches to the starter solenoid.
Tighten all bolts with a wrench, not a screw driver - a screw driver won't get them tight enough to make a good connection.
The main fuse may look good, but the ignition switch might not be sending it power.
Check for power at the fuse box with a test light.