One of my orchid's has just dropped the last flower off one of it's spikes and I wanted advice on cutting it back. It is growing another flower spike already.
Now please shoot me (or tell me off sternly) if there is advice for this somewhere glaringly obvious but I did get to page 12 of the Phal abuse thread before I got bored and I hunted through the general care pages and couldn't seem to see anything.
Quick answer: cut the stem just above the second growth node from the base. If the plant is healthy, it will send out a new branch from that point. If you already have a new primary inflorescence emerging, you can cut the old one down near the base, being careful not to cut into the leaves. Secondary branches are never as floriferous as the primary but healthy plants will support them.
I would also add some advice some wise person gave me on the forums here regarding cutting spikes, "If it's brown, cut it down. If it's green, don't be mean." That helps me remember what to do when I see spike's drying up on my phals.