After watching plenty of eps of Law And Order: SVU and various other crime dramas and also researching forensics on the internet, I have gleaned the tactics that police will use to solve a wide variety of crimes. And in every case solved in these crime dramas, the perpetrator forgot to cover their tracks in one way or another. While highly unlikely, it would in theory at least be possible for a person to commit a "perfect crime", which is to say a crime that can not be solved with modern forensic techniques.
Below is a list of tactics that police will use to find criminals, and how to beat them.
Tactic: Fingerprint ID
How to beat: Wear latex gloves.
Tactic: DNA ID
How to beat: Wear latex gloves, and use a vacuum cleaner to suck up any hairs you may have left behind.
Tactic: Photo ID
How to beat: If there is a place that is likely to have CCTV security cameras, case the place prior to commission of the crime, so you know where the cameras are. When committing the crime do one or more of the following for disguise: wear dark sunglasses, a hoodie, a wig, cut your hair, dye your hair, keep your face downward (most security cams are ceiling mounted).
Tactic: Witness ID
How to beat: Use one or more of the disguise techniques listed above if you will commit a crime where others may see it.
Tactic: Gun shot residue detection on suspect.
How to beat: After firing a gun, leave the area as soon as possible to avoid being coated with a microscopically thin layer of gun smoke. Also since your hands are closest to the smoke when the gun is fired, after firing a gun, wash your hands VERY THOROUGHLY with soap and water, as soon as possible.
Tactic: ID from scrape marks on lead slugs
How to beat: Never use the same gun for more than one crime. Throw it away after firing it in the commission of a crime.
Tactic: ID from serial number on gun
How to beat: File off, or acid erode off, or acetylene melt off, the serial number on the gun that you plan to use in a crime.
Tactic: ID from shell casing due to firing pin impact shape (and some other things too I think)
How to beat: Never use the same gun for more than one crime, or make sure EVERY shot fired from the gun has its shell casing accounted for and disposed of.
Tactic: IP address
How to beat: If the internet will be used for or for the furtherance of commission of a crime, make sure to send your communications through as many proxies as possible, preferably all located in foreign countries, preferably each in a different country, and preferably in countries that don't have "extradition treaties" with the US (so that they can't be legally forced to give up info on their users). Using VPN also helps as it means your computer isn't directly in the line of communications but rather is controlling a remote computer to access the desired website(s), and sending you the results of this browsing (text, images, even whole web pages) using a different protocol than the standard HTTP protocol. The remote computer in a VPN is (like with proxies) most effective when it is located in a foreign country, that preferably does not have an extradition treaty with the US.
Tactic: ID based on a collection of "little things here and there"
How to beat: Shoe prints, clothing fibers, etc can all lead to an ID, so before you go to the place you wish to commit a crime, change your shoes and other clothing to something you don't need to keep (vacuuming the floor if there is a vacuum cleaner nearby is also a good idea). After committing the crime, keep the old clothes until night, then BURN the clothes preferably in a dumpster in a dark alley so people won't see you going and burning the clothes, and while the fire department and possibly police arson units will arrive on the scene, you will be long gone before then. Make sure this dumpster is at least several city blocks away from the place where you live.
Tactic: Rape Kit
How to beat: Should you decide to commit this type of crime (and I pray you never will), it will be best to thoroughly scrub the victim clean (both inside and out) in his/her bathtub to get rid of all the DNA evidence (a suspect in one episode of L&O:SVU actually did this, but later got caught from other investigative tactics).
Tactic: Response to 911 call
How to beat: Make sure nobody sees you, and if you are directly attacking someone (such as for a kidnapping), do it stealthy (Navy Seal style) by sneaking up BEHIND your victim. First move when you get within striking distance should be to VERY QUICKLY reach over their head from behind and ductape their mouth (should already have a piece of ductape in hand when approaching) or put a rag soaked with chloriform (knockout gas) over their mouth and nose (This is a quick KO in about 2 to 5 seconds, at least in the movies and crime dramas). This will meant the victim can't scream to alert other nearby cops, or civilians who could call 911, nor can they call 911 themselves.
Note that it is VERY hard to pull off a true "perfect crime". So far, there is only one person who has done so, and that person still hasn't been caught, nor is their name even known. This is the person who raped and killed Jean Benet Ramsey. 2nd place for this goes to the Green River Killer, who for DECADES evaded law enforcement, but finally was identified as Gary Ridgeway, and arrested (and I believe executed, but I'm not sure if he's been executed or just serving life-in-prison).
Disclaimer: This post in no way means that I think committing crime is good. It is NOT good. I do NOT condone crime. I myself have NEVER committed a crime (other than downloading ROMs, warez, and pirated anime). This post is NOT intended to instigate any crime. It is designed ONLY as an educational post, to provide information (particularly to those who are fans of crime dramas or fascinated by the field of "forensics") about forensics currently used and the defects in those tactics that could potentially be exploited by criminals. If someone reading this post does follow through and commit a crime using these tactics, even in the event it was reading this post that caused them to firmly set their mind to committing the crime, I am not to be held legally responsible (in either civil or criminal court) for the actions that they commit, because of the fact that I am not writing this with the intent to induce any person to commit any crime. Also, since it is purely intended to convey information, the nature of the post falls under the legal classification as "educational", and therefore is protected as free speech under the First Amendment of the US Constitution.