Sash knot
Here is a superior way of knotting the sash of a bathrobe, house coat, or kimono.
Ashley's Book of Knots (ABOK) [1] identifies two knots for a
sash, ABOK # 1223, and ABOK # 1224 . ABOK # 1223 is tricky to tie and
untie. ABOK # 1224 is easy to tie and untie, and is a good knot for the
purpose. Both of these knots are difficult to tie snuggly around the waist.
If we base a sash knot instead on ABOK # 1474, a knot that Ashley
does not mention as a sash knot, we get a knot that is easy to tie to any degree
of snugness around the waist, and is easy to untie. As secondary benefit, we
learn (and practice often) one of the best knots for tying packages or joining
two pieces of twine. Here's what Ashley has to say about ABOK # 1474. [2]
"The Drawing Bend, Harness Bend, or Parcel Bend [The various names by which
Ashley had found ABOK # 1474 to be known. DMD] is about the most practical bend
for twine. There is no danger of capsizing as there is with the Weaver's
Knot, and it is very secure. It has an added feature which makes it invaluable
in parcel tying: it may be tied tightly while under tension."
A bend is a knot that joins two ends of rope. Of the three names Ashley gives for
ABOK # 1474, I prefer "parcel bend".
The parcel bend may be modified to
make it into a sash knot by slipping it -- by pulling through a bight rather
than an end as the final step, making it into a slip knot.
Procedure for tying the bathrobe knot.
Start with the sash passing behind your back. Tie the sash across your front
by the following procedure. While looking at the following pictures, imagine
that you are looking down at your front and are tying the knot around your waist
by knotting it in front of your body.
Step1. Place the end of the sash coming from the left side of your body,
1, over the end of the sash coming from the right side of your body, 2.
Step 2. Loop 2 up and over 1 and under itself toward the right.

Step 3. Pull 2 back over itself to the left to form a "crossing knot" in 2.

Step 4. Snug up the crossing knot formed by 2 and pinch it around the leftward
extension of 1 with the fingers of the left hand, leaving 1 straight as it passes
through the crossing knot in 2. Grasp 1 in the right hand. Pull to the right
with the right hand tightening the sash around the waist as 1 slides through the
crossing knot in 2. The grip of the left hand keeps the sash from loosening when the
right hand stops pulling on 1.

Step 5. Keeping the sash from loosening by the grip of the left hand, tuck a bight,
3, of 1 under the rightward extension of the tight sash. The diraction of
the tuck will be upward against your body.

Step 6. Push the bight, 3, through as shown. Pull the bight 3 with the right
hand to tighten the knot.

Step 7. The finished sash knot. Pull on 1 to untie.
References
[1]
Ashley Book of Knots
[2] Ashley, op. cit. , p. 267