Generally, most clematis do not need to be "anchored" to a trellis -- they will attach themselves naturally via twining leaf petioles. All I have ever needed to do was to unstake the plant after planting and gently guide the stems towards the support -- the plant did the rest :-)

Attaching stems can be tricky. With young or new plants, the stems are very tender/brittle and can break or become damaged very easily. If you feel you do need to attach, I'd use that stretchy garden tape that is some type of plastic material rather than the more crunchy twist ties -- less chance of damage.

Amending a clematis planting hole is generally well advised, contrary to what is common for most other plants. But you do need to be careful when doing so in heavy or clay soils, as you want to have very good drainage. Often, amending an individual planting hole in a clay soil will result in a 'bucket" effect, in that the enriched hole retains too much moisture and doesn't drain well. Amending a very large hole can help with this but the best result will come from amending the entire planting bed or area rather than just the planting hole. If this is not possible, creating a mound or berm or planting partially above grade will work, still keeping the base of the vine and the roots at the correct depth.

Thank you Gardengal. I assumed I had to attach it to the trellis in some manner so it's good to know that I don't! I've also read that it can help if you plant the clematis at a slight angle toward the trellis; it will encourage it to grow around the trellis.

Do you know how wide it will get? I'm still trying to decide between a fan shaped trellis or a rectangular one.

I dug up the flower bed this weekend and realized that in addition to it being quite clay-like there's also a ton of small rocks mixed in. Awesome. However, and I should have made this more clear in my first post, I will be amending all of the soil with a combination of peat moss, compost and garden soil. I just thought I might add a little extra to the hole that I dig for the clematis.

I have used string a few times on a trellises and fences, and it does help the clematis. The vines cover up the strings in a hurry.

Re: attaching it...if you have Dollar General stores in your area, in their garden area I saw this rubber-coated wire hanging on a little card.

At first, it looked like the fat cloth-covered rubber bands for hair (for ponytails), ya know? They come on cards like this.

But this is one long continuous piece coated with light green soft rubber (instead of cloth) and I'm guessing you cut it to length. Was only a $1, I think.

I meant to pick some up just to have but forgot....LOL