When agreeing to a request, agree to it in a positive manner. Don't just say 'Ok' or 'All right.' Use these positive phrases:

Yes, I'd be happy to.
No problem.
That should be OK.

Sometimes, you may be undecided and unable to give a definite answer at that moment. In such cases, use these phrases to buy yourself a little time:

Can I think about that?
I'll get back to you. Let me have a think.
If you don't mind, can I give you an answer this afternoon?
Give me some time to consider it.

At other times, you may agree to a request but with certain conditions. Then you can use these phrases:

OK. But only with the following conditions:
Yes, that's fine. But only if...
Sure, but I'd prefer it if you...

Declining a request is more difficult. Don't decline a request directly. Use one of tentative the phrases below and follow it up with a good reason:

I'm afraid I can't.
That's really not possible, I'm afraid.
I wish I could but...
I'd really love to help you, but...
I'm not sure if that's a good idea.
I don't know about that. You see...