Thanks for your question. I think the main ingredient to successfully growing agaves and yuccas in our local area is good drainage. The wording of your question shows you already know this. So, what I suggest is that you improve the drainage even more. I gardened in the SW for 10 years and lost plants there. The challenge you've given yourself by growing desert plants in Oregon is going to be a continuing learning exercise. Ultimately, the plant will tell you, show you what you need to do.

You can achieve good drainage by planting on a slope, in a raised bed, or in a naturally dry, sandy area. Or, you can attempt to duplicate excellent drainage with rocks, sand or gravel, or use pots (unglazed so moisture can evaporate).

Soil amendments may work for a while, but ultimately clay prevails! I would not add organic matter to the soil because it could slow drainage. The cactus soil mixes can temporarily improve drainage for a new planting but to repeat: clay prevails. Succulents with roots in cold, damp clay for months at a time are not happy succulents.

Here's what I would recommend:

1.  Plant shallow; most agave and yucca roots are shallow.

2. Till and amend the underlying clay with sand and gravel so the roots can permeate it if they desire.  

3.  Set the crown high. Never bury it. Not with mulch or anything. Do not plant in a flat or recessed area where water will gather.

4.  "Water-in" a new planting just enough to get air pockets out of the soil. Then, leave it alone. Add supplemental water sparingly. When you water, water deep and infrequently. Succulents die more often from too much water rather than too little.

5.  Watch during rain showers to make sure the drainage you imagined is happening. If not, increase the slope or add drainage pathways to accelerate the drainage.

Another suggestion: keep succulents in pots with a loose, fast draining soil (here the cactus soil mixes can help) until they are thriving. Then transplant to the good draining spots in your yard at the beginning of the growing season, keeping the roots undisturbed. Dig a hole that is at least 2 times the width of the pot, same depth as the pot. Center the plant in the hole, keeping the crown high. Spread the roots. Fill the rest of the hole with a 1:1 mix of the soil you removed and high draining material like crushed rock and sand. Water-in the plant and tamp down the soil.

If you don't have a slope, you can build one using rocks and soil.

Sedums also need fast draining, gritty soil.

Oregon State University has a publication that talks about succulents. You can view it at this link: http://extension.oregonstate.edu/gardening/stumped-over-which-succulent-grow-go-native

You have taken on a difficult, but potentially rewarding challenge. Good luck.