There aren't as many stringent requirements as fashion modeling, for example; you do not have to be a size 0 and 5'10" to be marketable. Since there are less restrictions, there is more competition.
What makes a memorable art model lies in other qualities-
-Punctuality is very, very important. Being 5 minutes late is not acceptable. If you're chronically late, especially for classes, you'll end up blacklisted and out of work.
-Preparedness. I always take a long t-shirt dress to wear for breaks, a scarf as a prop just in case, a timer in case the studio/ artist doesn't have one (happened once!); also, bring your own water and snacks. Turn off your phone!
-Charisma and demeanor. Even if you're having the worst day of your life, don't bring baggage. You can, however, put that energy into your poses. Be confident, even if you're having a "fat day" or a breakout. Nobody cares, I promise.
-Body awareness. I highly recommend taking a dance class, yoga class, or any other activity where you become more aware of your body.
-The ability to hold poses for extended periods of time. No matter how pretty and nice you are, if you fidget, "sag", or otherwise can't hold a pose well, that will annoy everyone trying to draw you. This ties directly into body awareness; know what is a good pose for your body, and what is hard to hold.
This is the most standard layout of a class I have encountered:
-Expect 2 to 3 hours for a class.
-Some classes, you hold one long pose; in some instances, it is acceptable to fall asleep in pose.
-Most classes, you will begin with a few minute-long gesture poses, then gradually move to longer poses.
-Many classes have a break time alloted.
-Always feel free to ask the director of the class about details of the class!
As a rule, I do the more daring, hard-to-hold poses first for short poses, and don't sit or lie down until the longer poses.
Make sure that you face everyone at least once! I generally face front, then move around clockwise until I've made a full circle. If I'm lying with my head one way, I'll switch which way my head is for the next lying pose.
If I do a lot of crunched up poses in a row, I'll change it up and do some elongated poses. If I notice people finishing their drawings early, I'll throw a real doozy of a pose. A "doozy" consists of anything hard to draw- anything contorted, foreshortened, abstract generally works.
In general, art modeling doesn't pay as much per hour as photographic modeling; depending on the local market, pay can range anywhere from $8.50-$50+ an hour for classes. If I can spend a week or more in a location, or have all expenses paid or enough shoots lined up, I try to art model as much as possible. There is more of an opportunity to book regular work as opposed to photographic work; a one-time high paying photographic gig though can give you the opportunity to travel a new location, and doing well at an art school or working repeatedly with artists can bring you back! Also, I have met many private artists through posing for groups and classes that do not use the internet and I would have never gotten into contact with otherwise.
