If you're carrying a pistol it must be in a holster. This could be a belt holster, ankle, pocket or a holster inside a purse. Regardless of what and where you carry, in order to do it safely it must be secure, inside a holster.
When the pistol is in the holster it is safe. The trigger is covered on modern holster designs, so the gun can't be fired. Remember Safety Rule II: Never let the muzzle cover anything you're not willing to destroy. With the pistol in hand you must constantly be thinking about muzzle control, keeping it pointing in a safe direction. When holstered you can stand, sit or crawl around on the ground without worrying about what direction the muzzle is pointing.
Always use a holster designed specifically for your pistol. No "one-size-fits-all" holsters allowed. The holster must hold the pistol securely – and protect it from the elements – yet still allow you to easily draw the weapon. You shouldn't need to use two hands to jerk the pistol free from the holster. The holster has to stay "open" after drawing the pistol, without the top mouth collapsing once the pistol is clear. This allows you to holster using only one hand. Since you're probably carrying concealed the support hand will be holding the cover garment out of the way as you holster the pistol. When it's time to draw, or holster, you may only have one hand due to an injury. Your holster must hold the pistol securely, allow you to draw smoothly, and reholster smoothly and safely.
Your holster must allow you to acquire the proper grip on pistol. Obtaining your firing grip on the pistol before drawing it is mandatory. Once it's out of the holster there may not be time to reposition the hand. This is important for holstering too. Just because you decided to holster doesn't mean the fight is over. You've decided to holster, but another problem appears. You're forced to get the pistol back into the fight. A properly shaped holster means you can maintain a firing grip when drawing and holstering.
The holster should stay in place during your daily activities. This is especially true for ankle holsters. Pocket holsters stay in the pocket as you draw. If the holster comes out with the pistol reconsider your choice in holsters. The smaller the pistol the more critical all of these requirements become. A smaller pistol is more difficult to grip, so it's essential the holster fits the pistol and your hand size.
The holster should also be stiff enough that you can hook the rear sight on the lip/edge in order to cycle the slide with only one hand. The holster is also used to strip mags free of the grip for Type III malfunctions. (This is also true of your belt, which may be used for this action using the support hand.) A good holster is used to manipulate the weapon when you're down to operating with only one hand.
Holsters do more than just hold the pistol. They keep the pistol safe, and protect it. With the right holster you can instantly draw, and safely, smoothly holster the pistol. The holster can be used to manipulate the pistol. The only way you'll determine if you've got the proper holster(s) is through training and practice. The good news is that today there are plenty of options. Don't be satisfied until you find "your" holster.
Tiger McKee is director of Shootrite Firearms Academy, located in northern Alabama. He is the author of "The Book of Two Guns" - http://shootrite.org/book/book.html writes for several firearms/tactical publications, and is featured on GunTalk's DVD, "Fighting With The 1911 - http://shootrite.org/dvd/dvd.html McKee's new book, AR-15 Skills and Drills, is available off Shootrite's website: http://shootrite.org/AR15SkillsBook/AR15SkillsBook.html