I only have 4 or 5 designs so far, but I always think about knives I've had or seen in the past. I might like the way the bolster was on that one and I like the way the blade was on this one, etc. I end up with a collaboration of features and shapes. No matter what you design, it's going to influenced by what you've been exposed to, so as long as you sit down and draw it out yourself, it's "your" original. One problem I have is size. I've got much larger hands than most people and I have a tendency to make designs feel good to my hand. Then I go to a knife show and see all those teeny tiny little knives and think, wow, maybe I need to downsize a little.
On another note, there are many knife pattern collections that the original designer freely shares. Some may ask for you to give credit to the original designer if you're selling the knife. Just make what you like, don't directly copy someone's work without permission, and don't worry about it.