It is because you tense up because you are being measured. I have my suspicions, it might be due to personality traits. Eg. Intense up before getting measured for anything, may it be blood pressure or an exam.
I think you are spot on there! Part of my reasons behind getting a monitor at home (besides proving to my doc that I don't need meds) is to get used to that "feeling". My BP sky rockets at a doctors office (>150/100) and I potentially have a medical exam coming up in a couple weeks so I want to make sure as hell that I make it through.
I intend to take a printout of my log, since I started recording my morning and evening BP, with so that he can see I am monitoring it and it is mostly in the pre-hypertension range (<140/90). Whether it will count for anything I don't know - he will have no reason to believe me that those logs aren't fake.
In the mean time this is what I am doing to help my BP:
1.) Drinking cayenne tea - 1/4 teaspoon 3 x per day. The first one in the morning I have mixed with the juice of 1/2 a lemon.
2.) Exercise - This week I will start a light cardio based workout routine. I had all intentions to start it two weeks ago but ended up in hospital with gallbladder issues. Holding onto it for now but it it's not better by Jan it will be coming out.
3.) Eating more fruit - 3 or 4 a day. Generally 2 x apples and a banana or kiwi fruit.
4.) Minimize salt usage - I haven't added salt to anything for the last 2 weeks. The wife still cooks with a bit of salt but I don't add more. It started off bland but as with everything you get used to it.
5.) Eat more raw foods and avoid junk food - Speaks for itself really.