I've worked a lot with singers in my life (ahimè...) and I've basically learned the technique without ever singing. well, actually, I did study a year just to see what it was like. Go123 says it basically the way it is. The higher you go, the more you want to tighten up and force it. It's normal. As a Bass, c is not that high a note. You know what it's like to yawn? Not just opening the mouth wide. The cavity of your throat opens up as well. Try it. Only the jaw lowers as you widen the throat and begin to yawn. Then, when you let out that note, try it with a nasal sound to it. Not that you should sing with a nasal sound, but to give you the feeling of the "maschera" - I don't know what you call it in English. The mask? Anyway, the area around the cheek bones and over your eyes. That's where the sound resonates.
Well, you can't really give a lesson here. But never ever force the sound with your throat.