Make sure you're using a very fresh blade, take your time, practice with it on scrap paper until you get the feel for it, and use a self-healing mat or glass cutting board. Adjust your pressure too, depending on what type of paper you're cutting.
Be careful and don't cut yourself! That's the advice I always have to remember.
Use a sharp blade, especially for small details.
I like to use a glass mat when doing intricate cutting.
Move the paper so that it's most comfortable to cut the part you are trying to do. Sometimes I forget this and seem to think that once I've put the paper on the mat, the paper can't be moved.
Relax your grip on the knife. I find that if I have a tight hold, my cutting isn't as smooth.
#1 for me (from experience)...remember to use my self-healing mat instead of my Tim Holtz non-stick mat
I move my design around more than I do the knife—especially on turns. I stick the sharp point into the mat, then swivel the paper to where it's a comfortable cutting angle.
Have good lighting so you can see all the intricate details.
If you're really having a tough time, maybe try a different knife. They all feel a little differently. Some people really like the little finger knife, but so far I haven't had much luck with it. I have a new exacto, but my best cuts are with an ancient, rusty exacto w/new blades. The weight seems to be different, so it holds in my hand better. Unfortunately, it's trial and error, but keep the receipts and maybe you can return what doesn't work for you when you give it a try.
Have fun...and remember, no one but you will know the parts that mistakenly got cut away
Great advice given above. I also love the fingertip control knife. I don't like the swivel one; it goes all haywire on me. However, the fixed blade one is fabulous. I love it. It takes regular exacto-blade refills, too. You don't need some funky blade that only fits in that knife, which is an issue I have with some manufacturers.
I am very clumsy with an exacto and never could use one well until I got the fingertip control one. With a coupon, you can get it for a couple of bucks!
I definitely agree about finding the knife/knives that work best for you. I have a few different types (including the fingertip) and I use them each for different types of cutting...sometimes it takes experimentation for me to figure out which works best on a project...or part of a project.
Also, practice the angle you are using the blade. For intricate stuff, I am usually using the very tip of the blade, but for long straight cuts, I have almost the entire blade (sharp edge) against the mat. I also hold the knife differently depending on the type of cutting I'm doing.
When you watch tutorials for different projects, pay attention to how they are holding and using their knife. I've learned some things that worked for me by paying attention to those details, even though it wasn't the point of the tutorial at all.