Tweak: If you have a Dexterity score of 13 or better, you can flank in a position other than standard. You can flank a creature when in one square offset from the position you’d normally need to be in to be considered flanking.
I’ve played an entire campaign arc using this tweak in DnD 3.5, and it worked out just right. However, that campaign was a rogue-themed story, and all the players were in need of additional flanking rules. You may wish to keep this tweak for a similar situation. However, it’s good for realism under any circumstances.