Taken from this answer on Stack Overflow: .htaccess mod_rewrite: rewriting querystring to path

<IfModule mod_rewrite.c>
    RewriteEngine On

    # Remove index.php
    RewriteCond %{THE_REQUEST} ^GET.*index\.php [NC]
    RewriteCond %{REQUEST_URI} !^/index\.php/admin\b [NC] # Don't mess with admin requests
    RewriteRule (.*?)index\.php/*(.*) /$1$2 [R=301,NE,L]

    # Remove ?p=
    RewriteCond %{THE_REQUEST} ^GET\ /\?(([^&\s]*&)*)p=([^&\s]+)&?([^\s]*)
    RewriteRule ^(.*)$ /%3?%1%4 [L,R=301]

    # Send would-be 404 requests to Craft
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteCond %{REQUEST_URI} !^/(favicon\.ico|apple-touch-icon.*\.png)$ [NC]
    RewriteRule (.+) index.php?p=$1 [QSA,L]
</IfModule>

This not only reformats /?p=about as /about, but it also preserves other parts of the query string.

For example, /?p=about&tag=stuff and even /?tag=stuff&p=about both redirect to /about?tag=stuff.

Taken from this answer on Stack Overflow: .htaccess mod_rewrite: rewriting querystring to path

<IfModule mod_rewrite.c>
    RewriteEngine On

    # Remove index.php
    RewriteCond %{THE_REQUEST} ^GET.*index\.php [NC]
    RewriteCond %{REQUEST_URI} !^/index\.php/admin\b [NC] # Don't mess with admin requests
    RewriteRule (.*?)index\.php/*(.*) /$1$2 [R=301,NE,L]

    # Remove ?p=
    RewriteCond %{THE_REQUEST} ^GET\ /\?(([^&\s]*&)*)p=([^&\s]+)&?([^\s]*)
    RewriteRule ^(.*)$ /%3?%1%4 [L,R=301]

    # Send would-be 404 requests to Craft
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteCond %{REQUEST_URI} !^/(favicon\.ico|apple-touch-icon.*\.png)$ [NC]
    RewriteRule (.+) index.php?p=$1 [QSA,L]
</IfModule>

This not only reformats /?p=about as /about, but it also preserves other parts of the query string.

For example, /?p=about&tag=stuff and even /?tag=stuff&p=about both redirect to /about?tag=stuff.