Best way respond to expressions of interest to sell site?

Every now and then someone will contact me asking if my site is for sale. I'm curious if anyone has had any training or experience in how best to approach these situations.

For example, I don't really want to be publishing how much I'm making to any old anonymous schmuck who comes knocking, out of vague unformed fears that info will be used against me / by competition somehow.

I also recall its best in negotiations to be the first person to state a figure, as the negotiation will happen around that number.

Adding to this, I'm doing some be refactoring which I hope will add a lot of value to the site, meaning my current monthly rev is not a good indication of where I hope it will be this time next year.