How do you say “no” in a tactful manner?

1- I can't relate to this now I have several other links

2- I wish I could participate in this, but...

3- I will think about it and get back to you

4- It doesn't fit my requirements now but I'll keep it in mind

5- I don't think I'm the right person to help you.. why don't you try (so-and-so)?

6- No, I can't do that

7- I would like to be with you but I will be later