Like i've said in other posts:
In Da'wah we call to 3 aspects insha'Allaah

:
1) Tawhid (Oneness of Allaah, His Lordship, none is worthy of worship but Him, His Names & Attributes.)
2) Risaalah - Prophets/Messengers who come to convey Allaah's message.
3) Al-Aakhirah. [The Hereafter - judgement day, paradise/hellfire]
Like brother
Kamal el Makki said, don't attack the other persons faith because they will hold onto the dirt in their hand tighter, instead show them your diamond (of islaam) and they will drop the dirt to hold the diamond instead insha'Allaah.
And like brother
Muhammad al-Shareef said: Build a wall for them first [of islaam] then they will realise the weakness of their own wall, so they will seek protection under yours because they understand its true security and strength.
Try getting a good understanding of them aspects, and then your da'wah will be alot more easier insha'Allaah because this way you'll be taking the drivers seat instead of the other person.
Here's some beneficial links for dawah insha'Allaah:
http://www.beconvinced.com/ http://islamtomorrow.com/site_map.htm Allaah Almighty knows best.
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