Note: I am not very PC when it comes to this sort of thing. I freely throw around words like cunt, etc (in case you haven’t noticed). If this offends you, you…might not want to read this post.
First, start with a body part. Obviously the “naughty” parts work the best for this, though “face” also has many applications. I use parts regardless of the gender of the person I’m insulting.
butt (sometimes it just flows better than ass–depends on what it’s paired with)
Then, take your ‘part’ and smoosh it together with another word. Often this word is random, though everyone has favorites. Face is one of those words that can pull double duty and go on this list, as well.
If you’re really cheesed off, you can always try adding descriptive words like “flaming”.