A model's reputation is very important in building her career. As a new model, you don't want to quickly build the reputation of being a "difficult diva."
The worst thing a model can do is cancel on a photo shoot at the last moment. If it was a job for a client, the photographer will still get paid, but the model won't. What's worse is if the photographer recommended you for the job, they now have egg all over their face and will usually refuse to work with that model ever again. You the model could even be sued for breach of contract and "shoot cancellation" fees.
Even if you're just late, your flakiness will cost you dearly. Don't do it. Always be on time and show everyone you're a professional.
Before accepting a job, make sure you know the details. If you agree to a gig and find that they want you teetering on a high rise roof when you hate heights, you're going to really hate yourself for accepting the job in the first place.
You should also request to see "roughs" the advertising designer or photographer has worked out. If it's a test shoot, request to see the photographer's work that is similar to the type of work you'll be doing so you have an idea of what to expect going into the job.
If you're not sure if you want to do the shoot or not, say so beforehand that you probably won't be interested but you'll think about it. That way, nobody goes and starts spending money in preparation for the upcoming shoot you never agreed to.
More information on modeling can be found at How-To-Get-Into-Modeling .com