I've read and tossed hundreds of shitty cover letters over the years, and read and written a few good ones. My unsolicited advice:
Do your homework about the company and the recipient. Tailor your letter to the specific position, but don't push it too far. Our job ads are pretty loose and informal, written in the company voice. I get a lot of letters that try to match that style or work in a specific phrase from the job listing. That can be a good thing, but it's a definite turn-off if it sounds forced.
Keep your letter as short and simple as possible. Edit, edit, and edit some more. Repeat.
Give your written voice a critical listen. I like a tone that's just slightly more formal than conversational. You want to sound confident and genuine, but still respectful.
I love it when people say "I'm seeking a position that will further my blah blah blah...", like I give a shit. What will hiring you do for me?
Use online samples as a starter, sure. But be sure to change up your letter enough to cover your tracks. For me, any letter that sounds canned goes straight to the trash. Execution is far more important than form, anyway. Guidelines that dictate which paragraph should contain what info are lame, IMO.
Have someone you trust proof your letter for spelling, grammar, and tone. If you that's not possible, proof the hell out of it yourself. But do get another set of eyes on it if you can.