The key to good straightening is going down the whole length of the hair. Going too slow will dry the hair out, while going too fast will have no straightening effect at all. If you just run smoothly down the length of a section of hair, you should avoid these nasty clamp marks :)
Same Thing Always Happened To Me, The Best Thing I Think To Do Is Make Sure You Dont Hold The Straighteners On Your Hair To Long, And If You Use A Small Amount Of Heat Protective Spraym, Your Hair Straightens Without Clamp Marks x
If you clamp down on the straightner when it is still on your hair it will leave a dent. Make sure your hair glides through the whole way to avoid these. Happens all the time to new users!
Start straightenning from the roots to the end and just glide the straightenners down your hair, if your straightenners are any good you don't need to keep them on one place for long