One "Trick", again in a long campaign, is to not upgrade to the latest kit (newly available tank, up gunned version, etc) until you lose the old ones in combat or change theatres (ie end of N. africa campaign).
This means that for Gemany, for example, you may still be battling on with outclassed PzIII when PzIVf/g, etc are available.
Also try using lower quality troops - Rather than SS, paras, Guards, etc use "Police" units, Home Guard, reservists, etc - lower morale and experience modifiers and older kit.
A third option is to intentionally not take some of the "essential" components of a combined arms force - ie no FO, or "forget" adequate AAA.
Judicious placement of Victory Hexes can help (try a delay battle where some, or all, of the VH are beyond your start line - meaning you have to move to take up ambush positions etc)
Just a few methods - looking forward to seeing more input from others
Re: Adding Some Difficulty
Hybrid Mode
