Even if you don't help the person with what they want from you, you could recommend a helpful book or another expert.
Even if you refuse to help, you can show that you understand the person's situation. Express understanding for how hard they work.
If you say "no" because of your busy work schedule, other obligations or circumstances beyond your control, explain it to them. You can also explain subjective reasons, such as your taste or style, if you think that the person should know.
Recognize that they turned to you. Thank them for their trust. "No" sounds much better when you treat others with respect.
Some people will come to you again and again to try to persuade you to help them. Psychologists call this the broken record technique. However, if you really don't have time or you don't want to help them for other reasons, don't change your "no" to "maybe". Be consistent.
The most advanced technique of expressing rejection is saying "no" with no explanation. However, even this can be said politely. Be kind and speak calmly. You don't have to apologize for anything. Saying "no" directly is more polite in the end than postponing the decision or making promises you can't keep.
-kk-