The current favorite is to adjust the AC chart and make all shields infinitely better. My only major problem w/ this is Armor/Power bloat. High level combats are going to take a lot longer. I'd like instead to offer an alternative: Treat Shields as a 2nd set of armor being worn(or armor being worn over natural armor), rather than as an additional modifier.
For instance, I'll use the 2nd edition shields since this is a 2nd edition blog: Buckler, Small, Medium, and Body.
You start by assigning each shield an AC you feel is appropriate: Buckler: AC 8, Small AC 6, Medium AC 4, and Body AC 2.
Next you put on your armor and shield. To do this you find the lowest(better) AC between the armor and shield. If there's a tie, doesn't matter which one you choose. The higher(worse) AC modifies this base AC by 1 point(in the favor of the armored character).
A few examples:
- Chain mail(5) + Medium Shield(4) = AC 3
- Padded(8) + Body Shield(2) = AC 1
- Full Plate(1) + Buckler(8) = AC 0
- Scale mail(6) + Small Shield(6) = AC 5