TRSE: How To Draw Tutorials:
Emotions:

Feeling Down, Disappointments:
Venus gets disappointed when she discovered that she missed her favorite TV show.
   The mouth usually gets lowered when disappointed as well as furrowing eyebrows.
 "Oh, I see...." -Mars-

"Why such long face?" would describe Mars's face when Moon told her that what Mars wanted was not available.
  The sinking of the mouth can be felt on the face, resulting in an appearance of "long face".

 Mars's in a bad mood as usual.

   This is a basic "Bad Mood" look usually accomplished by a black cloud above the head or purple haze steaming from the head/body as typical in Anime shows. Small & floating candle-like flames can be included depending on the situations. The eyes also vary too.

 "I'm so tired and sleepy." -Mars-

  When a person is sleepy, the person will look weak throughout the face and body.
 

.
Impatience, Frowning and Suspicious
Jupiter was feeling really antsy/anxious.

  Including the ants in the background provides an amusing exaggeration of how impatient and anxious a person is.
 

 Uh oh, Sailor Pluto's frowning!

  This is a basic frown that has the eyebrows lowering to touch the eyes as well as down-tagging of the ends of the mouth.

Moon was getting suspicious that we're up to something.

   Suspicious look usually result in a frown with narrowed eyes.

"What's this smell?" Moon

  That alerted look would happen if smelling/hearing something unusual/suspicious.

.
Sweatdrops:
The "sweatdrops" over the heads usually happen when a person gets dismayed or sometimes nervous.
  For example, Pluto was dismayed when she saw something she didn't like.

 

"Not again." -Taffe (JA's own character)-
 The sweatdrops also can accompany the "exaggerated sighs" uttered by the people.
.