My mom and dad are also packrats. (Not surprising I guess since both sets of grandparents were too.) DH and I have been cured of it by living in NYC where we don't have room for junk. Others are right, you can't convince someone else to declutter, but I do gently encourage it.
I require only that there be a safe place for us to stay if they want us to come there. (The first time we stayed there with dd it was horrible, thankfully she was too little to get into anything, but there were boxes of stuff literally up to the edge of the bed and the bed frame was falling apart.
) That at least collected their clutter so the livingroom and one bedroom were manageable.
I regularly try to engage my mom in conversations about what is in one of her many boxes, what she'd like done with them, and if I can help. Oftentimes it's a box of stuff she's saving for me or dd, and I take the box for her. Then I go through it, usually get rid of most of it, and keep the one or two useful things in it. A few years ago, I started refusing to let my parents just buy everything we're getting rid of, which they used to do.
I also asked my mom if I could have a garage sale at her house and she's joined in, so that should help.