First of all, go easy with the tools. I don't know what you are doing wrong with the seat post. Maybe the seat post is too narrow.
If you hammer your cotter pins, make sure that you are supporting the crank arm on a brick or piece of wood. If you don't do this, you will surely damage the bottom bracket. This also makes it easier to tighten the pin.
To stay tight, the cotter pin has to have the right shape, which you can obtain by filing it down to look like the original one.
Ideally, you should use a cotter press to tighten the pin, but you can use a heavy duty C clamp instead. This is what I do.
Don't even think about trying to tighten the pin with the nut. It'll strip right away.
Read Sheldon's page on cottered cranks.