Someone recently sent me an email asking to use of my murder mystery party games. Sadly my notes are in no way, shape, or form coherent enough to be used by someone other than me. However! I did come up with this pretty thoughtful response, which I thought you on the internets might enjoy. Take this as my apology for not blogging in so long... or don't, like I care.
If you are thinking of writing your own murder mystery, here's
 the formula that has worked for me (still in 
development): 
1. Start with writing the setting and figure out who will be your victim (usually played by the author) 
 
2. Come up with your suspects - three people with reasons to want him or her dead
 
3. Figure out three alibis and/or provide exonerating evidence for all three
 
4. Compromise one of the alibis/pieces of evidence (usually via your supporting cast) 
 
Casting is often the biggest challenge. You need three people 
who will do the extra work of being suspects and one person willing to 
take on the job of solving the murder - the detective. I sometimes 
ask certain supporting cast members to be "allies" and give them a bit of 
information that may be helpful in solving the mystery. Duplicating 
clues (providing the same information in more than one way) will 
probably speed things along. In my experience, if I think my clues are way 
too obvious they are probably just right. 
A few other thoughts - You'll probably want to have other 
games/activities going on while the detective figures out the mystery, 
cause people will get bored. These can be tailored to fit the theme, and
 can help maintain the fantasy (along with decorations and costumes). 
Themed food is also fun, but can get labor-intensive. Anyway, hope this 
helps!