I am a rank beginner having run my MH in Senior exactly twice. One stupid thing I've done... don't point the gun at the judges. Otherwise, be there when the judges explain the test and watch the test dog. When you come to the line, the judges will ask you if you understand the test, if you're not sure, don't be afraid to ask.
I will tell you something that a friend told me she used to do... night before HT, she writes out a 3x5 card. It has stepwise the things she wants to remember. She goes over it in the holding blind (because it gives her something to focus on instead of getting wound up.) Can't remember exactly what she puts on it, but it's something like.
* walk to line
* sit dog
* tell judge my dog's number
* get instructions from judge
* tell dog to SIT
* call for birds and shut up
* WAIT for number
* Take a breath
* Send dog (She writes dog's name on card because sometimes she's running more than one)