Crime scene tape and chalk outlines are an easy and great way to decorate these parties. Be sure to make them life like in size by having someone actually lay down for you to draw the outline. You can leave false footprints, fingerprints, and clue cards all through the area (not too well hidden unless you want people snooping). You might even get lots of help as people will more often than not enjoy helping set up a crime scene and it makes for something unusual. You can incorporate multiple rooms and your garden as well, if you are feeling truly adventurous, have a different case going on in different rooms and have the kids draw numbers for teams and race to solve the crime first.
You might also do well to set up a lab, complete with microscope and tables dedicated to sorting out the evidence and make notes for the shocking conclusion of the crimes in question. Always take into account your guests - children should, obviously, have simpler clues whilst you can make an adult party a little more tough. Use your favorite episode for inspiration if you must and throw in a red herring or two to keep things fun. Young and old alike, the kids will have a good time quarrelling over the clues and trying to find out who really committed the crimes (and of course making up their assumptions and stories as to why the crimes occurred in the first place).
For food you can have an actual CSI cake or go with cookies that are shaped like footprints or thumb print cookies for an excellent play on words. Of course by the time children are old enough to enjoy a party such as this they probably would be happy with good food rather than themed food and pizza, sloppy Joes, finger foods, chips and dips, and cookies and cake will make an excellent food addition to the party without needing to fit in with any one theme. Be sure to have plenty of pop and bottled water on hand though because crime solving is thirsty work.
In real life it is best to leave the jobs of solving crimes up to the men and women who were trained to do so. In the world of birthday parties however, crime solving and crime scene investigating reign supreme as the next great thing in birthday party ideas.