The fixform trick is neat, but:
You may not have access to the code of what loads in the new window.
Even if you do, you are depending on the fact that it always loads, error free.
And you are depending on the fact that the user won't click another button before the other page gets a chance to load and run fixform.
I would suggest doing this instead:
OnClientClick="aspnetForm.target ='_blank';setTimeout('fixform()', 500);"
And set up fixform on the same page, looking like this:
function fixform() {
document.getElementById("aspnetForm").target = '';
}