It's all too easy to say 'YES' when you're starting out on your career, when you're eager to get work and make an impression, but sometimes 'NO' is the better word.
I see a lot of adverts on the web looking for experienced screenwriters for features offering no payment, only a credit, a DVD copy of the finished film and if you're lucky an invitation to the wrap party. But why should you give your time and talent for free? Presumably the producers/directors who place these adverts are expecting to make some money from their film, so why aren't they offering you a percentage of the producer's profit at the very least?
Unfortunately, there are too many writers out there happy to be taken advantage of and work for free. What you should be aiming for is a deferred payment so that when they get their production budget in place you actually get to see some money. At the very least, as I've said above, you should be offered a percentage of the producer's profit. If you don't get either of these then walk away and let some other mug get taken for a ride.
This doesn't just apply to new writers. There will be times in your career when you'll be asked to do something for free, because the producer doesn't have any money to pay you for another draft, or be asked to make changes to your screenplay you disagree with, or be pressured into unrealistic deadlines. This is where you have to make a decision as to whether what you're being asked to do is worth while.
Let's take being asked to do another draft for free because the producer doesn't have the money until he gets funding. Should you do it? What you need to ask yourself is, 'do I believe in this project and does it have a good chance of actually getting made?' If the producer is on the level s/he won't mind agreeing a deferred payment for the work. If they do mind then alarm bells should be ringing and you should seriously consider walking away.
What if the producer/director wants to make changes to your screenplay you don't agree with? If these changes make the screenplay better then you really shouldn't have any objection to them, but if the changes significantly alter your screenplay for the worse then you should think about saying 'NO'. It's always better to state your reasons to the producer/director why you don't think the changes will work and ask them what their thinking behind them was. It may just be there is another way you can help the director get what they want without ruining your hard work. Negotiation and finding compromises are the key here, unless of course the director wants to introduce dancing bunnies in wellies to your period drama because they saw it on TV once and thought it was funny. Then it's probably the right time to scream 'NO' at them, so loud it bursts their eardrums.
What about unrealistic deadlines? Again negotiation is the key. If you really can't write a second draft of a 120 page screenplay in 3 days then tell the director/producer the time you think it will take. If you can't come to an agreement then don't say 'YES' just because you don't want to be kicked from the project. What's better; to agree to meet the deadline and fail magnificently and put your reputation on the line, or walk away by agreement with no hard feelings?
'NO' isn't a word you should be scared of. Always be willing to put yourself out if need be, but also be prepared to say 'NO' if you feel you should.