A friend and I have devised a relatively decent and structured way to pick our armies. It isn't ground breaking, it's being done the world over I am sure, in other ways, but I thought I'd share it and see what folk think.
(1)We generally break the game size down into Small, Medium or Large.
(2) For Small:
50 Wounds cap.
No Warscroll can be picked more than once.
1 Character Warscroll (the only compulsory)
0-1 Artillery
0-1 Ranged unit (to dissuade campers. This is a very alterable rule if you think it's unfair, e.g Wood Elf, but it does attempt curtail 40 hand gunners sitting on a hill.)
Unit Size - Whatever you want, as long as it meets the minimum and doesn't cause your army to exceed the maximum of 50 wounds.
To go to Medium or Large scale, simply double / triple the restrictions, respectively. This also applies to the no warscroll can be picked more than once. So, in a large game, for example, you may have 3 Warscrolls of knights.