Raised beds were the best thing I ever did for my poppies, because it allowed me to amend the soil as I saw fit, and it provided the perfect balance of moisture and drainage. The reason I started growing in raised beds was because the soil where I live (while great for corn), was too hard for poppies. Even when I tilled the ground it didn't take long before it got compacted. So, it was partly due to laziness that I built the beds.
When building a raised bed you should find a location that suits the plants to be grown in it. After I built my first bed I realized that it was too close to my privacy fence, essentially shading a third of it through the first half of the growing season. This was because I built it later in the summer when the sun was directly above. So that second year I marked were the shadow fell throughout the season and built my second bed to get full sun.
Selecting the materials you use to build your bed are also important. Do not use any lumber that's been treated. Use landscaping timber, natural stone, or clay brick. It used to be thought that railroad ties were safe if they were old and weathered, but the truth is they will always contain dangerous chemicals that will seep into your soil. I've been told that concrete blocks (like the ones used in the construction of building foundations) are treated with "something", but I'm not a chemist or a mason. The safest bet is to check with the manufactorer of the landscape material.
When constructing the frame, reinforcement is sometimes necessary. Boards will bend once you start filling it with dirt. Rebar stakes can be driven into the ground next to any flimsy boards to add strength. I've even seen people make a wall with dirt they just dug up, like an earthen dam. If you have clay like I do, that can work, as long as it drains well.
Growing Medium (Soil):
A good "cannabis" soil mixed with extra perlite, coarse sand, or very fine stone works well. I currently use Fox Farm, but I started out with an inexpensive generic brand to get the beds filled. If you live in an area (like I do) that has any amount of clay in the soil, adding sand is a death-trap. Clay + Sand = Concrete. Avoid using topsoil or gardening soil which will compact over time, and hinder growth. Poppies have a main taproot that needs loose soil. Too much moisture can lead to stunted growth, root rot, and downy mildew. Warmer climates may find you watering a couple times a day, so maintaining good drainage is crucial. Over watering and over fertilizing are the two most common mistakes. Poppies actually do well with little water, and I never fertilize mine. I find that mixing compost into my beds at the end of a season is enough to feed the plants for the next season. Because poppies are drought tolerant, they also do well in containers. A 3 to 5 gallon greenhouse planter is an ideal size for a single plant. I found an old wash tub at a garage sale, that I filled with potting soil and it adorns my driveway for all my neighbors to admire (at a distance, my dog makes sure of that). I have had some success with growing poppies in hydroponics (drip method, no flood&flow) under 400 Watt HPS grow lamps, but nothing beats Mother Nature.