Jean Millay, who wrote a fabulous book about ESP and remote viewing called Multidimensional Mind: Remote Viewing and the Evolution of Intelligence, has made a list of things which can detract from your ability to remember accurate images. My images, I've found, are almost always somewhat tainted by this life, yet they retain a kernel of truth within. For instance, Thomas Carter does not exactly look like my description or drawings, but there is a similarity:
Here are some of the things Jean Millay says can alter the past-life visions you receive and make them less than accurate (which I've put my own spin on, by the way, since she wasn't talking about regression in her book, but instead ESP and sending/receiving target pictures -- but the mechanism is the same):
1) Things in your immediate environment; a dog barking in the neighbor's yard may cause you to come up with an image of a dog in your past life that really wasn't there.
2) Your own self-confidence; you might not have enough faith in your own abilities to trust that what you are seeing is real, and you then dismiss the image or feeling.
3) Anything you think about often in this life can influence what you see in a past-life regression and taint it.
4) Sleepiness, which inhibits reception of any ESP-type phenomena.
5) Resonance with the sender, and an emotional relationship between you, is crucial to receiving ESP; so, too, you must have resonance between this life and the past-life you are trying to remember.
6) Your mood can influence the mood of the past-life visions you see and taint them.
7) Memories in this lifetime can taint your self-regression images, and pop up instead of past-life material.
8) Dream-like symbolism that has to be interpreted, and is not taken literally, can confuse your self-regression.
It's important to remember that, although many past-life memories are primarily visual, those same visual elements can be tainted by passing through the filter of your current personality. Almost nothing gets through without being contaminated at least somewhat by your current concerns, likes, and thoughts, yet there is nearly always some core truth to these memories. Practice can help weed out the false information, and bring the true past-life visions into focus. If you're looking for a method for regressing yourself and remembering who you were, check out my page about past-life self-hypnotism and self-regression.