Common reasons for saying 'yes'...
1. It seemed easier at the time
2. She made it feel like I couldn't refuse
3. Guilt, just guilt
4. She made me feel like it was an honour.
5. There seemed to be mysterious hidden consequences awaiting those who said 'No'
6. I couldn't think of an excuse quickly enough.
These are all fair enough, but... well you know the buts. You spend time on stuff which does not contribute to your long-term goals. You lose your carefully constructed sense of being in control. You are more likely to feel resentful or anxious. You have less time in your immediate future than you thought you had; you will have to replan. There is a greater risk of work piling up, risking stress.
Several aspects here:
1. Don't say no to everything.
You need to get used to routinely estimating tasks, and asking how much work is involved if it isn't clear. Don't say 'no' to trivial or reasonable requests for help (if it doesn't impact on your plans). Build up a stockpile of goodwill.
2. Don't say yes to everything.
You do want to be seen as helpful and a team player; you don't want to be seen as a pushover or a dogsbody. If it seems like you are being taken for granted - this is frequently evident in the tone of voice or lack of thanks - then a polite, smiling refusal is a good move.
3. Follow your instinct.
If you're not sure, follow your instinct. If your gut says 'no', say 'no'.
4. Fix the Guilt thing.
You were brought up to be polite and co-operative, I am sure; consequently you may instinctively feel guilty at refusing to help someone - it seems rude. Think this through and get over it. You have nothing to be guilty about, you are entitled to decide on your own priorities and to put those above others priorities, especially when the well-being of your students and the goodwill of your family may be involved.
5. Practice saying no
This is not natural and needs practice. You should always start with the word 'No'. Then explain using a positive-negative-positive formula. Examples (with positive elements in blue):
No. I know this is important, but I'm already busy. I'm sure you'll be able to get other help.
No. It sounds great, but my kids are expecting me home. Ask me again if you do it next month.
No. I wish I could help, but tonight's out of the question. Hope it goes well.
No. That's lovely, but I really must finish the marking. I appreciate the offer, though.
At first, the word 'No' might seem incompatible with the rest of the formulation but it does come with practice.
6. Meet persistence with persistence.
Some people never give up. Make sure you're one of them...
- Sorry, I can't stay behind on Wednesday
- Wednesday is out of the question, I'm afraid
- My kids really look forward to Wednesdays, so 'No'
- Impossible on a Wednesday.
- etc...
7. Use non-verbal softeners.
Smile, nod, empathize, while saying 'No'. Especially smile. Be pleasant, while being firm.
8. Handle your boss carefully
Make sure you understand what your boss (HoD, Head) is asking you to do. If it is a reasonable request within the scope of your job description, then 'no' isn't a valid answer, unless you can justify it. Ask for clarity regarding how much work is involved, how frequently you will need to do it, and why you - specifically you - are being asked to do it. Be prepared to negotiate.
9. Use your boss
If you are put on the spot by powerful stakeholders (parents, governors) then play for time, and seek support from your boss. He/she gets paid more to say 'no' to people, and should be aware of how much work you already have. If your boss says 'yes' to this, seek a reduction in your workload elsewhere.