As well as entitling you to respect and, hopefully, a wage, professionalism also includes a commitment to getting a job done once you take it on.
Some projects can take longer than expected (listen to the presentation by Alex Julyan on rates of pay; a project meant to take 6 months ended up taking three and a half years) but it is important to get as much of the project agreed in advance, preferably in writing (even if only an email) so that all the project partners – you, the other artists or organisations you are working with – know what to expect, and what tasks will be done by whom. This also includes how much you will get paid, and at what stages in the project.
Through contractual negotiation you can ensure your project will take place roughly as you want it to, and you should negotiate as much as you need before signing any contract / letter of agreement or agreeing to begin. It can be useful to have your own standard terms and conditions of employment that can be sent to any prospective employer – set you own agenda and negotiate, even if the organisation you are working for does not suggest this.
It is important to know your own worth and have a rate of pay in mind for when projects arise and you need to have an idea of how much your time costs. This rate will vary depending on the project and how well (if at all) it is financially supported.
More information on contracts can be found in the Artlaw section of this site.