Any advice on what to use other than milk?

Use sour cream! It will make those mashed potatoes absolutely delicious - rich as all get out, but so, so good (exactly the kind of decadence you love in mashed potatoes).

For 4 russet potatoes, I'd start by stirring in 1/2 cup of sour cream, then I'd taste it, adding more if I felt like the potatoes would benefit from more. They probably would. Oh, and would you mind inviting me over to help you eat them? I'm sitting here, about to leave my office at the end of a long week, hungry as can be, and I can't think of anything I'd rather be eating right now than those mashed potatoes! ;o)