For those of you brave enough to admit you tie and fish flies that incorporate squirmy-wormy material, you know what a pain it can be to getting it lashed to the hook. It wants to roll, smush, sever and just be a general nuisance. If tied too tight, it will simply fall off while left perfectly alone in your fly box.
To begin, do yourself a favor and use a heavier thread that can be flattened, like UTC 140 Denier. Prior to tying in the material, give your bobbin a gentle counterclockwise spin to uncord and flatten the thread. This extra surface area helps prevent the thread from cutting into the material during tie-in.
Pass the squirmy material underneath your tying thread twice then take a thread wrap around just the hook shank in front of it, followed by a posting wrap around the base of the material. End with a single wrap around the hook shank, behind the material.
You can then put a little tension on the material and start taking ever-tightening wraps of tying thread rearward. As you do this, give your bobbin a clockwise spin to cord up, strengthen and thin the thread. This will allow you to cut through the excess material as opposed to snipping it off. The advantage of this method is the material gets gradually bound down and is less likely to break off.