Ah, the elusive bell pepper. It's awkward shape and strangely stubborn skin. Cutting this veggie stumped me for a while until I was shown a better way by a fantastic chef in New Zealand. If you need precise cuts, say for garnish or to ensure even cooking times, this is the way to go. Yes, there is more than one way to skin a cat, but this method has always been my favorite.
Using a serrated knife, "top and tail" the pepper, slicing off the top and bottom. This helps to create a nice rectangle to work with.
Stand the pepper up on one of the flat sides, and make a slit all the way down one side.
Lay the pepper sideways, and use a sawing motion to work the serrated knife around the inside of the skin and around the seeds. Discard the middle.
Cut the long rectangle into two smaller, more manageable pieces if you wish.
Switch to a sharp chef's knife. Place the skin side down. Using a rocking motion, slice the pepper into strips (1/4" thick for small dice and up to 1" thick for large dice).
Gather a few of the strips and turn them so they are now perpendicular to the edge of your knife. Slice across the strips to create a dice, adjusting the thickness as desired.
You can repeat steps 5 & 6 for the tops and tails of the pepper to avoid waste...just understand that your cuts will be less even.