I've been carrying a folding knife clipped to my pocket for almost 20 years. My first was a spyderco with a crappy, non-removable plastic clip - super awesome for the time, not so great with all the amazing folders that are available these days. For all of these years, I have almost exclusively carried my knife tip down. It felt more comfortable, like I could get my knife out and deploy it more quickly that way.
Recently I've been watching some videos online and some videos by Michael Janich.
The Janich video Fighting Folders was excellent and showed me that I needed to give tip up carry another try. My deployment from the pocket had been all wrong. I had been grabbing at the exposed part of the handle (like my years of tip down draw) and then trying to choke my hand up on the handle while sort of hitching the knife down, an ineffective method - which is why I never used it. The correct way for fast deployment is to put my thumb deep into the pocket and use my index finger to pull from the end of the pocket clip. Pulling this way puts the knife in perfect position for an immediate opening, faster and much more comfortable than my tip down draws. Works amazingly well with my small hands and the relatively new knife I have been carrying, the super grippy, short-handled, very awesome Blackhawk Hornet II. Not sure if it would work as well for me with a longer folder, I don't have anything suitable to test though.
The only downside to the new carry method is the constant practice needed to get comfortable with it. I am constantly drawing the blade, trying to simulate drawing under stress, awkward positions, etc. Unfortunately, I got clumsy yesterday and took a piece out of my thumb on the edge - OUCH! I did a short video of my clumsiness that you can see here - feel free to laugh at me :)
How do you carry your knife?
Note added May 1, 2011 - After a bunch more test draws, I've figured out how I cut the hell out of my thumb - it's the dual thumb stud. On a very infrequent basis, the stud is catching on my jeans enough to actually open the blade slightly. When doing a fast draw, my thumb automatically moves into position and pushes on a slightly open blade, OUCH! I've never run into this problem before as I always carried my knives tip down. Not sure what I want to do here, can I grind the thumb stud down on the left draw side? That's the part that is catching....hmmm...this is such a shame, I love the knife but I'm going to have a hard time trusting it. That knife is VERY sharp, my cut could have been even worse with more pressure.
This gives a reason to consider a Spyderco blade - no thumb stud to catch on clothing. Except that I don't like that enormous spyderhole bulging out the blade.....more hmmmm.....I'm waiting for some mini AK-47's from Cold Steel to show up, going to try them out - they actually do this on purpose! At least I'll be ready for it!