I always kind of chuckle at this advice. Is there any case of a representative changing their mind on an issue because someone wrote to them about it? I've written to representatives in the past and you are always going to get one of two outcomes:
1. If you are writing in support of something the representative already agrees with, you'll get a form letter back saying: "Thank you for your input. Representative Jones fully supports initiative X and is fighting to enact it in Congress!"
2. If you are writing in support of something the representative disagrees with, you'll get a form letter back saying: "Thank you for your input. While your input is valuable, Representative Jones is adamantly against initiative X and will not be supporting it in Congress."
I think of lawmakers as unchanging boxes of fixed beliefs and you only get to vote for and change the contents of that box every N years. The real way to combat this is to elect representative who already align with what you want.