I will just say “no” to avoid over-committing myself and set boundaries

For some of us, saying “no” to avoid over-committing ourselves is very hard. But in order to address the stress that builds up from over-committing oneself, it is wise to consider some of these tips when you are feeling over whelmed with requests. Following are some tips on how to just say “no”:

First, don't be afraid to say no. Honoring a desire to rest or hold off from doing something you can’t give 100% can be a good choice to make.
When responding to a request, be respectful and polite but FIRM. This shows that while you’re sympathetic, your mind is made up.
Keep your answer short. “I’m sorry, I’m not available then” or “I have another commitment”, is enough.
Do not lie when you explain why your answer is no.
If your refusal upsets someone, remain calm and change the subject or compliment them somehow. Exiting may also be an option to consider.
Practice saying it in the mirror if you’ll feel more comfortable and able to say “no”.

Setting appropriate boundaries is another important way to avoid over-commitment and feeling overwhelmed. Following are some tips for setting boundaries:

Say “yes” when others ask to help! If someone volunteers to help, or someone asks you if you need anything or someone asks you if there is anything they can do, or buy, or clean, or change, or take care of for you….say yes
Say thank you — if others DO help, just say thank you. Not, I'm sorry I needed you. Not, I feel so bad that you had to do such and such. Just thank you. Thank you for being my friend, thank you for taking such good care of me when I needed it, thank you for caring, thank you for understanding me or my situation so well. No apology. No regret. No defensiveness. No shame. Just thank you.
Be kind to yourself and to others. Try to smile even when you don't feel like it. You might just trick your brain into thinking you feel good.
Be realistic: Dial down your expectations. Settle into the here and now and accept your current state. Breathe deeply.
Express yourself: bottling up your worries, your feelings, your concerns and opinions is a sure-fire way to sabotage your well-being. Talk to someone you trust. Speak up if you feel misunderstood. Do not tolerate toxic feelings or relationships.
Drop the guilt: none of the above will be possible if you surrender to feelings of doubt or regret. Keep a sound bite in your head, "everything will be okay." Or, "all is good." Repeat, over and over again, until the guilt has nowhere to settle.

Adapted from: http://www.psychologytoday.com/blog/isnt-what-i-expected/201112/10-tips-setting-boundaries-and-feeling-better