I am not a professional florist but I have learned some tips through trial and error and through floral decorating workshops I have attended over the years. It is here that I am sharing some of these tips with you so you can have fun creating your own.

Tip #1
I always follow the THRILLER, SPILLER, FILLER method when designing an outdoor planter. The THRILLER is the part of the arrangement that provides the height and is meant to make a 'statement' in the arrangement. The SPILLER is the part that spills over at the base of the arrangement. The FILLER fills the space between the THRILLER and THE SPILLER.
In my opinion there are no set rules and it is up to you what to use for your THRILLERS, FILLERS and SPILLERS. Just use your artistic license and creative ideas.

Tip #2
If the plant pot sinks into the planter use a paving stone or a brick to add some height.

Tip #3
Make your fall floral arrangement in a separate plant pot and then insert it into the planter. It makes it easier to pull out the plant pot and dismantle the fall floral arrangement especially if the soil is frozen. (I have to think about that where I live).
Tip #4
Place a grapevine wreath wrapped with ribbon on the rim of the planter. I use this as a SPILLER and it nicely hides the gap between the inserted plant pot and the planter.

Tip #5
Make use of natural foliage. Here I kept the dracaena from the summer planter along with some tall branches to use as the THRILLER. I water the dracaena until the soil freezes. After that it stops growing but still continues to look really good in the arrangement for several months.
Tips #6
Make use of artificial silk floral bushes and berry sprays. I inserted two fall themed floral bushes for the FILLER and some berry sprays along the bottom of the arrangement for the SPILLER.

Tip #7
Add what I call a showstopper to your arrangement to finish it off. For this arrangement I inserted three metallic spray painted dried giant alliums that I got from my garden. Adding a big bow works too.
Tip #8
To keep costs down look for items to add to your arrangement on sale and out of season, in thrift stores and from your own garden.

I hope the photographs and tips inspire you to make your own outdoor fall floral arrangement. I actually make three of this design every year and place them in the front in between the garage doors. It is quite doable and a lot of fun to make for even the non-professional floral arrangers like myself.
I invite you to visit the blog where I share more details and photographs!