95 Cavalier wont start, Help Please

I've had my 95 Chevy Cavalier for almost 4 years now and have had the same problem the whole time. I turn the key and I hear a clicking (and soem hummung- fuel pump I think) but it wont start. It can take me as much as 50 times trying it before it finally starts. And then sometimes it starts up just fine on the first try. I know it's not the battery, the lights and the radio and everying come on. I've had the ignition switch replaced, had the starter repalced, and gone through 2 new selenoids. Once it starts, it runs perfectly fine.
Today, for the life of me, I couldn't get it started at all. For 10 minutes I kept trying over and over. The same clicking noise and humming, but would not fire at all.
I am at my wits end with this. Please Help.

How do you know it's not the battery? Some batteries can have dead cells.

First off, I'd completely clean off your battery terminals.

Second, check/replace your spark plugs & spark plug wires.

Third, you may have a defective crankshaft sensor or PCM valve. Or you have just have to clean them.

After that, check all the wiring to the solenoid & starter. My guess is the spark plugs.

How does it run once you get it started???

I'm not sure but I hope that I'm not to late on this post... In front of your car underneath around your starter you should see a bunch of wires and are bolted down by a bolt or nut.. Inspect these wires and make sure that none of them are broken.. If all seems fine loosen bolt or nut and take a piece of sandpaper and clean off all wire ends and also clean the area where they are connected to...
We have ran into many problems with Chevy cars with your problem and solves the problem but the inspection of these wires!.
Hope this helps.... Take Care.

Just a small note that I forgot to say.... These wires are not the wore connected to the starter solenoid... They are connected to the engine block or transaxle housing!