In the Name of Allah, most Compassionate, most Merciful

Shahadah (The declaration to faith) is the most misunderstood statement among the Muslims today.  The Muslims of today use bits from hadith here and there and simply imply that just by declaring the shahadah from the tongue, a person is guaranteed paradise!  If those ignorant and Islamically uneducated Muslims study the Quran and Sunnah/Hadith carefully, they will not reach this conclusion.  If in the time of the Prophet (S), the pagan Mushrikeen leaders of Makkah took the meaning of shahadah as the Muslims do today, then they would have all accepted Islam with a big smile.  They would still bury their daughters alive, drink alcohol, adultery, theft, and all the sins possible under the sky and above the earth even after saying the shahadah.

So what guarantees Paradise?  I will give examples from Quran and Hadith to clear the common misunderstanding and deception.

Quran 23:1-11. Successful indeed are the believers. Those who offer their Salat (prayers) with all solemnity and full submissiveness. And those who turn away from Al-Laghw (dirty, false, evil vain talk, falsehood, and all that Allah has forbidden). And those who pay the Zakat. And those who guard their chastity. Except from their wives or (the captives and slaves) that their right hands possess, for then, they are free from blame; But whoever seeks beyond that, then those are the transgressors;  Those who are faithfully true to their Amanât (all the duties which Allah has ordained, honesty, moral responsibility and trusts etc.) and to their covenants; And those who strictly guard their (five compulsory congregational) prayers (at their fixed stated hours). These are indeed the inheritors. Who shall inherit the Paradise. They shall dwell therein forever.

A person is guaranteed paradise in this verse of Quran only if the person does what it states.  Now if a person wants to do the above, it surmounts to basically learning the whole Quran and Sunnah and its implementation and understanding the above things which Allah listed.  If we look in a number of hadith we find that similar statements are in effect.  I will note a few.

Al-Tirmidhi Hadith 4870 Narrated by Ubadah ibn as-Samit
The Prophet (peace be upon him) said, "If you guarantee me six things on your part I shall guarantee you Paradise. Speak the truth when you talk, keep a promise when you make it, when you are trusted with something fulfil your trust, avoid sexual immorality, lower your eyes, and restrain your hands from injustice."

So this hadith basically says the same thing as what Allah has ordered.  The promise and trust mentioned in this hadith apply between the Muslim and Allah as well, not just among the people.  

Sahih Al-Bukhari Hadith 8.481 Narrated by Sahl bin Sad
Allah's Apostle said, "Whoever can guarantee (the chastity of) what is between his two jaw-bones and what is between his two legs (i.e. his tongue and his private parts), I guarantee Paradise for him."

So the thing we must note is that many things need to be accounted for as a guarantee to paradise.  We cannot take an isolated hadith, or bits of hadith from here and there to come up with such a big conclusion that saying the shahadah alone guarantees paradise.  As in the following hadith...

Sahih Muslim Hadith 50 Narrated by Abu Hurayrah
"...Now the first one I met was Umar. He asked: What are these sandals, Abu Hurayrah? I replied: These are the sandals of the Messenger, of Allah with which he has sent me to gladden anyone I meet, who testifies that there is no god but Allah, being assured of it in his heart, with the announcement that he will go to Paradise. Thereupon Umar struck me on the breast and I fell on my back. He then said: Go back, Abu Hurayrah. So I returned to the Messenger of Allah (peace be upon him) and I was about to burst into tears. Umar followed me closely and there he was behind me. The Messenger of Allah (peace be upon him) said: What is the matter with you, Abu Hurayrah? I said: I happened to meet Umar and conveyed to him the message with which you sent me. He struck me on my breast which made me fall down upon my back and ordered me to go back. Upon this the Messenger of Allah (peace be upon him) said: What prompted you to do this, Umar? He said: Messenger of Allah, my mother and father be sacrificed to thee, did you send Abu Hurayrah with your sandals to gladden anyone he met who testified that there was no god but Allah, and being assured of it in his heart, with the tidings that he would go to Paradise? He said: Yes. Umar said: Please do not do it, for I am afraid that people will trust in it alone; let them go on doing good deeds. The Messenger of Allah (peace be upon him) said: Well, let them.

So in this hadith, we clearly see that at the end, both Umar (R) and the Messenger of Allah (S) agreed that people cannot depend solely on shahadah alone to go to paradise and would end up in hellfire.  In this hadith, Umar had the type of people in mind as found now a days, so that is why he made Abu Hurayrah go back and stop saying this.

And lastly, the above hadith and the main issue of this topic is even more clarified from Prophet Muhammad (S).

Sahih Al-Bukhari Hadith 1.50 Narrated by Abu Jamra
"Do you know what is meant by believing in Allah Alone?" They replied, "Allah and His Apostle know better." There upon the Prophet said, "It means:
1. To testify that none has the right to be worshipped but Allah and Muhammad is Allah's Apostle. 
2. To offer prayers perfectly
3. To pay the Zakat (obligatory charity) 
4. To observe fast during the month of Ramadan.
5. And to pay Al-Khumus (one fifth of the booty to be given in Allah's Cause).

So from this hadith, we can see that anything Prophet (S) use to say, even very small statement, it means something major and big, and not taken literally but studied and understood properly.  Here even the statement "belief in Allah" means so much.  And the five things each have sub meanings as well which need to be learned and understood. 

So the guarantee of paradise is only through learning the Quran and Sunnah properly and then implementing it as mentioned in the above hadith and verses.  If a person doesn't do it, not only he/she can risk going to hellfire, but even risk going there forever for doing something major haram mentioned in Quran and hadith.