Plugin input sanitization

Ok, I'm banging my head on something that should be really simple.

I've written a simple plug-in that has a settings page with a few textboxes and a textarea.

When the settings are saved, I want to sanitize the input (per best practices) and have tried various sanitization things like:

htmlentities( $_POST['myvariable'] );

The problem is that strings like "I've read this before" get changed to "I\'ve read this before" and if saved again becomes "I\\'ve read this before".

What am I doing wrong?