I've been using rubber bands for my new Java Fern and Anubias, they work well. The Anubias I bought came potted, so I pulled the rock wool off the roots, placed an elastic band around the middle of the plant and gently stretched it around the wood. It's been a few weeks, and the Anubias is putting out new leaves, but I don't think it's secured itself yet.
[edit] I did some maintenance on the pico today, and checked the Anubias - it's securely attached to the decoration. I looked it up in my diary, I've had it about a month.[/edit]