In your .htaccess, try
RewriteEngine On
RewriteBase /
RewriteRule ^(/?([^/]+/)*[^/]+)\.aspx$ $1 [L,R=301]
Now in every URL that ends with ".aspx", this ending should be cut off.
If this doesn’t work, you can at least see where you’re directed to when you look at the URL field of your browser. Post the result here, and there’ll surely be someone here to help you (I’m going on holiday tomorrow morning

).
If you just want to rewrite a few addresses, you could also do this one by one:
RewriteEngine On
RewriteBase /
RewriteRule ^(/?about/why)\.aspx$ $1 [L,R=301]
RewriteRule ^(/?about/why_not)\.aspx$ $1 [L,R=301]
RewriteRule ^(/?blog/this_is_why)\.aspx$ $1 [L,R=301]
You could also rewrite all URLs of specific directories:
RewriteEngine On
RewriteBase /
RewriteRule ^(/?about/[^/]+)\.aspx$ $1 [L,R=301]
RewriteRule ^(/?blog/[^/]+)\.aspx$ $1 [L,R=301]
I hope this helps...
Cheers,
Jan