The only good rule is a broken rule, or at least that seems the consensus of most of fandom.  In that vein, the following suggestions should be considered not rules, but pretty good ideas which may save your life.
     There are two types of costumes: competition costumes and hall costumes.  The former
appear to have been created by F/X artists, and are equal or superior to what you see in most Hollywood movies.  They are designed to be worn on stage, before an audience, and for a relatively brief time.  A hall costume, on the other hand, is something you go partying in.  If you go to a convention you may wear the thing all day.
     That may be true of some parties, as well.
     In designing your hall costume, there are some things you should keep in mind, to insure both safety and comfort.  First, and most often neglected, is the issue of vision.  If you wear a mask, it should have large eye holes.  If you are using a veil, make sure that the material over the eyes is either transparent or very nearly so.  And if you are rated to wear glasses, put them on. It's all very well to say that you can see in your costume while standing in a brightly lit dressing room, but if you must walk through the dark of night in your costume, or if you are in a dimly lit room, and especially if you are on stage in a costume parade, you will find that the slight impairment of your sight will become a major one.  Then it is a slight misstep off the walk, step, or stage, and down you go to crackling accompaniment of your bones.
     It's a real bummer to a party to have one of the guests taken away in
an ambulance.
     Likewise, you should be extremely careful of pieces trailing dramatically from your costume: giant wings, billowing capes, elegant dress trains, etc.  They are beautiful on stage -- leave them there -- but are completely unsuited for the crowded conditions of the hallway or party.  Other people might step on or tangle up in them, and they can brush against burning candles or bonfires. And then the host has to call an ambulance and the fire department.
     Another great danger to be wary of is overheating in a costume. Wearing a big rubber suit is dramatic, but unless you have an electric fan and a water bottle in there, you are in danger of heatstroke.  Another way to overheat is through the overuse of body paint. Apparently skin suffocation is a real thing (Remember GOLDFINGER) and you can die from painting your entire body.  If you wish to come to the party in nothing but a paint job, remember to READ THE INSTRUCTIONS and leave certain critical areas of your body uncovered.
     Safety issues aside, there are certain comforts to consider in your
costume.  For example, well, maybe Rigelians don't have to use the facilities, but after a few
toasts of Bhlog you most certainly will have to.  Your costume should be designed with...access.  I once was wearing a little fairy costume, consisting of a cloth belt, a lame shirt, a leotard, tights (because the shirt was so short) and various undergarments.  I spent half the day in the bathroom dressing and undressing...

    And you should have access for eating and drinking, as well.
     Also, your hall costume should afford you a full range of movement. Can you walk, turn around, and fit into the elevator while wearing your costume?  I remember one dramatic costume, where the man was dressed as a space shuttle.  He was working a table in the huckster room and had to stand there, all day long, as his costume did not allow him to sit down.
     Don't burden yourself down with unnecessary paraphernalia, either.  At the most, should have only one thing in your hands, so that you can hold the drink glass with the other.  Preferably you should be carrying nothing, in order that you can hold your plate as well.  If you feel you must have a pet dragon or exotic object to complete the outfit, then I suggest you find some sort of bag or pack that you can wear over your shoulder or tied to your waist.  Or perhaps you can hire a  child to follow you everywhere, carrying your parcels for you like a small, servile elf.
     Peacebond all weapons.  You really don't want then getting into the wrong hands, such as those of the police officer who happens by.  Make sure that your costume has a pocket or some place to put your driver's license, for you won't be getting any bhlog without it.  And if you are wearing this costume at a convention, remember that you will need some place to pin your badge. (Another problem with body paint, unfortunately.)
     Last but not least, be aware of the sensibilities of the other guests. Some perfumes and potions have strong scents which get only stronger in a overheated room, and are very bothersome to those of us with allergy problems.  Religious items should be worn respectfully. Manners are always a good addition to any costume.  And don't forget -- peanut butter, when warmed by body heat, does turn rancid.  (Entirely Helen Davis for the 1999 celebration.  I am always tempted to add a few comments to this, having met Helen, but I will restrain myself to saying that she is the mother of a pair of non-servile twin elves.  Catherine Mintz)