Crime scene tape and chalk outlines are an easy and great way to decorate these parties. Be sure to make them lifelike in size by having a person actually lay down for you to draw the outline. You can leave fake footprints, fingerprints, and clue cards all through the area (not too well concealed unless you want people snooping). You might even get lots of help as people will typically enjoy helping set up a crime scene and it makes for something unusual. You do not have to limit the revelry to one room and you can, 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, this area can be used by the teams to sort through their evidence and make notes for the conclusion of their investigation. Of course you will want to make the crimes easy and solvable by leaving clues that the kids will have no difficulty finding or deciphering without making it too easy. 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 arguing over the clues and trying to determine 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.