You have to be perfectly comfortable with your partner. I know this might sound ridiculous, but your body will just open up and get wet once it makes the subconscious decision that you're comfortable. This is a problem with married couples, too. I know this is true because out of all my partners, only one was able to make me get wet at all, and we didn't even need foreplay. It was just being comfortable in his presence and TRUSTING him fully. I don't think anyone on this site can "cure" the tenseness, while all the tips are useful, they just don't cure the problem. You might have to talk to a professional or explore why your body might not be comfortable (even though you think that you are). Good luck!