How deep is it and what sort of soil do you have as a native soil, ie. sandy, clay, gravel? What will you be planting?

Bed will be 8-10 inches deep. Just planting flowers. Tulips, petunias, whatever. Currently the bed is empty but local soil which I could glean from various parts of my yard is fairly sandy with a LOT of gravel in it.

To avoid textural interface issues, which can impede drainage, it would be best to use a sandy soil to fill the planter. You'll want to stay away from anything that has a significantly higher silt and clay content than your native soil. A little sandier would be okay if that's what you can get your hands on.
Since the bed is only 8 to 10 inches, you can use organic amendment throughout the full depth. I personally would use an 80% mineral soil to 20% organic material blend. You mentioned peat, which would work well. It's a little pricey but very stable and will result in less settling than something like a compost. (bark fines are also relatively stable) The down side of a sand/peat blend would be low nutrient value but you could top dress with good quality compost or use fertilizers to improve soil fertility. Any sort of fertilizer rates or efforts at pH adjustment would depend on the chemistry of the final blend.

When there is a lot of gravel in the soil, it can sometimes consolidate and compact, which can impede drainage. If the native soil below the bed is severely compacted, it would be best to break it up before installing the planter soil.

Wow. Thanks. Looks like it's time to start learning all this stuff.

A lot of folks use what is often called Mel's Mix.
1/3 peat moss
1/3 compost
1/3 vermiculite

If you are going to use a fair amount of your yard dirt, I agree with gargwarb...mix in a fair amount of organic material such as compost and peat moss.