Knot tip

A cool knot for securing a fixed line on something like the raps from the Heart or down from Sickle, or any two bolt anchor is the Double Figure 8.

It is fast and secure, and gives you two quasi equalizing "ears" to clip into the bolts.  Here is the link for a demo of how to tie the knot:  Double Fig. 8

Rappelling with a skinny trail line

There is a discussion going on over at SuperTopo that is directly related to a rappelling fatality in Yosemite last week. There are many posts on the technical aspects of rappelling with one lead line and one "pull" line of a small diameter, when the anchor consists of rap rings.

Petzl has a diagram of how they suggest a single line rap should be done.  Their method is safe enough to keep you alive, but it too has problems with the knot possibly getting through the rap rings and then becoming un-retrievable from below .  I've suggested that the clove hitch method be used and is the safest method available in this scenario.
The relevant technical posts start here in the thread.
There is some really good info in that thread and is worth your time to read it and understand it.
Clint Cummins has just added a great presentation of Yosemite style anchors and which rap set up will allow or not allow the knot to pop through the anchor rings.