Relatively plain evergreen wreaths are ubiquitous this time of year; consider a trip out to a local Christmas tree farm, where you can buy wreaths at good prices made from locally grown cuttings. Trader Joe's also has some good-looking, inexpensive options, but I do think it's nice to support the farmers in your area if you can.
Once you've got your plain base and a few simple supplies, it couldn't be easier to dress up your wreath for the season. You'll want to start with a bunch of the little wooden wonders called "wired picks" (pictured above). It's basically a thick toothpick (or a miniature wooden stake, like the kind you'd use to kill a vampire) with a length of wire attached to it at the top. You can get them at any craft or floral supply for super cheap; here they are at $1.99 for 90, which is way more than you'll need for a single wreath.
Essentially all you'll need to do is lay the stem of whatever material you're working with flush along the length of the pick and wrap the wire around it. If you need a bit of extra support to keep your stems from breaking, use some floral tape wrapped around both stem and pick.
Then insert the pointed end of the pick into the bundled greens at the center of the wreath. If you look at the back of your wreath, you'll see that most likely it's wired onto a frame; you want to aim to place your pick in the center of that frame. If it's too tightly bundled and the pick won't go all the way in, try pushing it straight down (again, vampire-killing style) instead of at an angle, or just get it wedged in the best you can, even if it's not between the wires. You'll find that with lighter plants and cut material, the picks won't budge even if they're not placed exactly in the bundle.
I use the picks for my tillandsias too, bending back a few of the lower leaves and sort of bundling them with the roots around the pick. You can do the same with little clumps of moss, or pretty much anything else you can think of. (Sometimes with moss you don't need a pick at all; it will suffice to just wedge the clumps in among the greens.)
Tillandsias are air plants (their roots grow in the air), and while they don't like cold weather, they make great houseplants anywhere at all. Remove them from your wreath at the end of the season and place them around your home in little dishes or vases; a weekly mist of water and some nice indirect bright light is all they need.
If you're using succulent cuttings, you might find that some plants are too heavy to attach to the wired picks. Try some thick-gauge wire instead, inserting the wire directly into the stem of the plant. I prep my succulent cuttings by removing all of the soil and thread-like roots, leaving only the stem behind. (If you don't have succulents growing in your garden, look for them at your local nursery.) After the holidays you can plant your succulent cuttings again in the garden or in a pot, just tucked into a bit of soil, where they'll happily take root.
The thick-gauge wire will be even easier to insert into your bundle of evergreens. Again, use floral tape wrapped around the stem and wire if you're finding that your stem breaks easily. When you're finished with your design, lift up your wreath and use a wire cutter to trim any pointy ends of the wire or wooden picks that are sticking out of the back of the wreath.
I like the look of a wreath with a cluster of decoration in one spot, leaving nice stretches of simple greens. But you can design your dressed-up wreath however you like. Consider using cut materials that dry well (like the pepper berries and moss we used here) to complement your living plant choices.
Have fun! And if you're in the Bay Area, come see me and my friendly co-workers at Flora Grubb Gardens. We've got all the supplies you'll need to design your own wreath, and you can even sit and work on it in the gardens with your coffee.