do you always clench your fist as hard as you can at the end of every punch? or do you clench as hard as you can before you throw the punch?

no, let your hands relax, and let the target close the glove... you will be more relaxed, and faster

I do it just before it lands or as it being thrown. And release as soon as its landed. You would not be able to clench it all the time as the gloves would cause it to ache after a while and your arms will tire quicker etc. You shouldn't clench as hard as you can just clench it to a fist with gloves on you won't be able to make a proper fist as you would without them on.

I know what i do, just wondering what others do :D

i try and stay relaxed the whole time..i usually kinda keep an open glove

I leave it open when I throw and clench it just before it lands. I shadow-box with hand grippers, BTW.

A tight fist at impact delivers a drastically harder punch.

i stay relaxed then tense at the last second.