a stupid battlemat trick

If you use minis and a markable surface, here's a fun thing to do when the party accidentally stumbles into the lair of Mr. Bad and his minions.  First, take a minute to write the numbers 1 to 30 on your battlemat, something like this:

Place terrain, if any.  It's totally okay if the terrain sits on some of the numbers.  Then set up the foes on the blank half of the map.  Make sure to use some miniatures that looks like they are going to mess the party up.

Now, for each character in the party pick a custom die throw for random placement.  Do you have a stalwart meatshield who always takes the lead when marching down smelly dungeon hallways?  Have that player roll 1d6.  Someone who always places themself in the middle of the formation might roll 1d6+6.  The PC who always guards the rear might roll 1d8+12.  Someone who stays with the group but rarely specifies their position might be a 3d6 or 1d20 roll.  The sneak thief who cares more about a nice safe, shadowy hiding spot than the party formation might roll 2d6+12.  And that one PC who is always manically running around everywhere and doing everything rolls 1d30.

Go around taking turns for placement.  If a player rolls a number that is already occupied by another PC, they may pick any empty adjacent square.  If the square is occupied by terrain, the same basic rule applies, though for some terrain types the player could opt to be standing on top of the terrain or maybe the thief started scrambling up the wall for some reason just before the monsters showed up.

Do this a couple of times for funsies then hit them with the bonus version.  Write a second set of numbers on the map in a different color, a few of which are adjacent to the party's set of numbers.  Then take turns with the party.  One player rolls and places, then you roll for one badguy, back and forth until the fight is already pre-trainwrecked by random placement.

Again, this is for when the party wanders into a big lair of bad guys.  I wouldn't use this method for a properly scouted and planned attack by the party against a known foe.