Why would you want to make a pin more challenging? There are many reasons to up the brutal on a machine:
1. 6 digit and too easy to roll
2. Make the game better for league play
3. Cut down on play times
4. Improve your save skills

This thread should have generic and specific ideas, settings and rules.

I'll start with a generic idea from @alwaysrockin777: Alcohol plays a big part. Now it's hard to mandate consumption but too many can make a game much harder.

I know some people like long play times but this is bad for league play.

So have at it, what are your ideas?

Here's specific tips for Bally Viking:
1. Tune the saucer to shoot straight down the middle requiring a saving move.
2. Tight Tilt
3. Remove the large rubbers from the outlane and use post rubbers to make saves harder.

One of the games we play at league is Swords of Fury and some players have started to do to well at it (long ball times too).

Some of the things done: Lightning flipper on the upper.
Remove the rubber from the inlane post.

But it remains a long playing game.

Here's one thing I'd like to try: tighten up the sling switches. The ball slides past the left sling when coming down from the upper playfield. Sensitive slings would put you in jeopardy and take to focus off of the upper playfield as a source of easy points.

It still gets rolled though!

interesting thought. Do they play like powerball?

some great suggestions here. What about game specific settings that influence gameplay?

I really like the simplicity of this approach.