I'm asked this a lot so this is my answer!
(also find this article at Pin-Up Parade)
1. Get a professional shoot done. If you've read anything about mainstream modelling, models need a portfolio of pics in order to join an agency. Pin-up is no different - in order to be seriously considered by other photographers (or indeed shops & magazines etc), they need to see samples of your work to see what you're capable of. Myspace-style photos in a bathroom mirror, or out-of-focus things taken on a self-timer with a basket of laundry in the background (when not intended as a prop, obviously) are not sufficient. When you're starting out, the best shoots are ones when you pay because you will be taught how to pose for the camera. For photographers who do this in the UK and Europe, please look at the Galleries at Pin-Up Parade and also the links page. I will put a list of photographers at the end of the page.
2. Please don't expect this to be a magical short-cut to a life of glamour and marabou-trimmed negligees and no more office work or standing in queues or going to Tesco's to buy baked beans. Not even Dita makes a living solely from "being a pin-up". Bernie Dexter has an online shop and sells vintage items on Ebay. Sabina Kelley works in a tattoo parlour. Morgana does web design. Some girls add the money from paid shoots to money from burlesque performances and costume-making and pin-up classes, but they are not solely working as pin-ups.
3. Which leads to... pin-up is a fun hobby! It really is! But it's not a wage-earner. You might get paid shoots, but it won't be enough to pay the rent.
4. Go back a step and think about what you mean by "getting into pin-up". Doing some pin-up photos and having fun doing the shoots? Being a celebrity? Having people cue up to tell you how wonderful you are? If you say yes to the first option, then bravo, you're being sensible. Fame and adoring fans aren't a given. Doing photos and enjoying the process is the basis of "being a pin-up".
5. As with all forms of modelling, there are limited opportunities for larger women. True, there are more opportunities than in other kinds of modelling, but do be aware that there are often size limits on the models required for some shoots, and if you protest, you're ear-marked as a trouble-maker. I rather enjoy being a trouble-maker myself, though.
6. Networking is important but don't make unreasonable demands on people. You aren't the only person who "wants to be a pin-up". I'm often asked by people if they can model for my shop, and I'm now having to say that you must have previous modelling experience. Even if it's a shoot you've paid for with The Hourglass or The Casting Couch. It isn't fair to bombard people with photos you took of yourself in your bathroom mirror and demand that you're hired. Don't leak bad karma into the cosmos! (man). You MUST have a professional portfolio!
7. Enter competitions, participate in online groups, etc etc etc. For example, The Pin-Up Lifestyle Network. Just being generally involved in burlesque or rockabilly, which have pin-up as important elements of their culture (though please don't profess to liking rockabilly if you don't really like it - it's kind of annoying). Come up with your own ideas for websites or online groups or forums. Whatever. Be creative. Don't expect everything to fall into your lap.
8. It might be worth joining Model Mayhem and/or Net Model, but as I've mentioned elsewhere, there are often rather unrealistically stringent size requirements so larger girls will find it very hard, if not impossible, to find shoots.
9. You don't have to perform burlesque as well. Shocking, I know, but it's true.
10. Learn how to do your own hair and make-up. This will open more opportunities for you, as it means the photographer won't need to hire a muah.
Photographers offering portfolio shoots:
(this isn't an exhaustive list!)
Nicole Klein of The Hourglass
Tony "Nylons" Rusecki of The Casting Couch.
Lisa Williams Cherry Bomb Rock Photography
Miss Giggles www.missgiggles.info
Spooky Sally at Cherry Muffins studios (website not working)
Danielle Bedics Photography
Atomic Cheesecake Studios
Mitzi & Co.
Viva Van Story
The Pin-Up Workshop Co.
(please let me know if you have a suggestion to add)