6mm (and smaller) WW2 models infantry in ranks :P
The following works for my group. We have tight games, and our winningest player plays Sisters of Battle. Presented with explanations.
1: Armies should be 40% Troops. So a 1000pt army should have 400pts in troops, a 1500pt army should have 600pts in Troops, and so on....
If you REALLY want to fix 40K, this is a good start. Troops are better balanced against each other than anything else in the game, and it prevents issues of playing against an army that invalidates large swathes of what you have in yours. Plus, it makes armies more thematically appropriate.
Flipside - this does generate issues with detachments that don't have Troops slots. The resolution is to pick a unit that should be the "Troops" of the formation - for instance, Ravenwing Bike Squadrons for the Ravenwing Strike Force.
2: Allies / Detachments / Formations
Armies must be comprised of at least one Battle Forged Primary Detachment. Army lists may not include Allies from Codices representing fully fleshed out factions. Army lists may include Allies from Dataslates, as long as they do not draw units from Codices representing fully fleshed out factions. Army lists may include Formations, which may be fielded as additional Detachments as necessary.
The Allies rules break the game more than anything, and in the first version of our house rules, they were out entirely. Then, I wanted people to be able to use Cypher, so an allowance was made - hence the stuff about fully fleshed out factions.
3: Flyers rules
We're going to handle flyers a little differently than what's in the rules. People with flyers, don't include them in your lists. However, at game time, you can tell your opponent you'd like to include a flyer. If your opponent agrees, you can include the flyer without altering your list. Your opponent then has the opportunity to add an equal number of points to his/her list. They can take any combination of flyer of their own, a unit with Skyfire, and/or a fortification with an anti-air upgrade, up to the amount of points the flyer you are using. No more than 1 flyer may be used per list.
We've got this because the distribution of flyers and anti-air is uneven, so a player doesn't have to cripple his/her army to have flyer defense just in case. Works pretty well.
4: Super heavy vehicles / Gargantuan Creatures
As with Flyers, do not include them in your main list. At game time, you can tell your opponent you would like to include one of these. If your opponent agrees, you can include the unit in question without altering your list. Your opponent then has the opportunity to ad an equal number of points to his/her list. This unit takes up your Lord of War slot on the FOC.
Same reasoning as the flyers, but as none of us own any super heavies/gmcs, it's been a moot point. Eventually, one of the Eldar players will buy a Wraithknight though, and it'll be on.
5: A player may only use the power dice from one psyker at a time for casting powers or for making dispel attempts. Dice from multiple psykers may never be combined for a single roll.
This does two things. One, it prevents a lot of the shenanigans the psychic phase currently allows. Two, it makes having a single psyker relevant against a psychic-heavy army - the single psyker still has a reasonable chance of being able to cast powers.
My Truescale Insanity
http://www.lounge.belloflostsouls.net/showthread.php?48704-Truescale-Space-Wolves